Move `test_index_is_created_for_both_timestamps` to `TimestampsWithoutTransactionTest`
Move `test_index_is_created_for_both_timestamps` to `TimestampsWithoutTransactionTest`

This commit addresses these failures when the backend RDBMS executes implicit commit for DDL like MySQL and Oracle.

```ruby
$ ARCONN=mysql2 bin/test test/cases/integration_test.rb test/cases/timestamp_test.rb --seed 58225 -n '/^(?:TimestampTest#(?:test_index_is_created_for_both_timestamps)|IntegrationTest#(?:test_cache_key_for_newer_updated_at|test_cache_key_format_for_existing_record_with_updated_at|test_cache_key_format_for_existing_record_with_updated_at_and_custom_cache_timestamp_format))$/' -v
Using mysql2
Run options: --seed 58225 -n "/^(?:TimestampTest#(?:test_index_is_created_for_both_timestamps)|IntegrationTest#(?:test_cache_key_for_newer_updated_at|test_cache_key_format_for_existing_record_with_updated_at|test_cache_key_format_for_existing_record_with_updated_at_and_custom_cache_timestamp_format))$/" -v

TimestampTest#test_index_is_created_for_both_timestamps = 0.19 s = .
IntegrationTest#test_cache_key_format_for_existing_record_with_updated_at = 0.05 s = F
IntegrationTest#test_cache_key_format_for_existing_record_with_updated_at_and_custom_cache_timestamp_format = 0.02 s = F
IntegrationTest#test_cache_key_for_newer_updated_at = 0.01 s = F

Finished in 0.272880s, 14.6585 runs/s, 14.6585 assertions/s.

  1) Failure:
IntegrationTest#test_cache_key_format_for_existing_record_with_updated_at [/home/yahonda/git/rails/activerecord/test/cases/integration_test.rb:111]:
--- expected
+++ actual
@@ -1 +1 @@
-"developers/1-20170717135609430848"
+"developers/1-20170817135609283207"

  2) Failure:
IntegrationTest#test_cache_key_format_for_existing_record_with_updated_at_and_custom_cache_timestamp_format [/home/yahonda/git/rails/activerecord/test/cases/integration_test.rb:116]:
--- expected
+++ actual
@@ -1 +1 @@
-"cached_developers/1-20170717135609"
+"cached_developers/1-20170817135609"

  3) Failure:
IntegrationTest#test_cache_key_for_newer_updated_at [/home/yahonda/git/rails/activerecord/test/cases/integration_test.rb:147]:
--- expected
+++ actual
@@ -1 +1 @@
-"developers/1-20170717145609430848"
+"developers/1-20170817135609283207"

4 runs, 4 assertions, 3 failures, 0 errors, 0 skips
$
```

`ActiveRecord::TestCase::TimestampTest#test_index_is_created_for_both_timestamps`
calls`ActiveRecord::TestCase::TimestampTest#setup` which updates `updated_at` column value.

```ruby
    @developer = Developer.first
    ... snip ...
    @developer.update_columns(updated_at: Time.now.prev_month)
```

This transaction expected to be rolled back, but if the backend RDBMS like MySQL perfomes
implicit commit when DDL executed, this transacion is committed by `create table` statement
inside `ActiveRecord::TestCase::TimestampTest#test_index_is_created_for_both_timestamps`.

It causes these failures above which expect `update_at` column value are not changed(rollbacked).

prathamesh-sonpatki/rails,eileencodes/rails,EmmaB/rails-1,kddeisz/rails,rafaelfranca/omg-rails,rails/rails,assain/rails,untidy-hair/rails,esparta/rails,tjschuck/rails,Stellenticket/rails,prathamesh-sonpatki/rails,iainbeeston/rails,deraru/rails,yahonda/rails,georgeclaghorn/rails,yhirano55/rails,yhirano55/rails,illacceptanything/illacceptanything,prathamesh-sonpatki/rails,fabianoleittes/rails,deraru/rails,printercu/rails,pvalena/rails,baerjam/rails,kddeisz/rails,starknx/rails,aditya-kapoor/rails,aditya-kapoor/rails,starknx/rails,kmcphillips/rails,assain/rails,palkan/rails,Edouard-chin/rails,travisofthenorth/rails,pvalena/rails,rails/rails,untidy-hair/rails,gfvcastro/rails,bogdanvlviv/rails,joonyou/rails,joonyou/rails,Sen-Zhang/rails,baerjam/rails,bogdanvlviv/rails,alecspopa/rails,rafaelfranca/omg-rails,Sen-Zhang/rails,mechanicles/rails,schuetzm/rails,kmayer/rails,felipecvo/rails,BlakeWilliams/rails,Stellenticket/rails,vipulnsward/rails,mechanicles/rails,yalab/rails,illacceptanything/illacceptanything,illacceptanything/illacceptanything,brchristian/rails,kmcphillips/rails,Stellenticket/rails,yasslab/railsguides.jp,yawboakye/rails,yasslab/railsguides.jp,kmayer/rails,eileencodes/rails,gauravtiwari/rails,baerjam/rails,joonyou/rails,lcreid/rails,tjschuck/rails,illacceptanything/illacceptanything,yalab/rails,Erol/rails,pvalena/rails,rafaelfranca/omg-rails,notapatch/rails,tgxworld/rails,Erol/rails,EmmaB/rails-1,iainbeeston/rails,illacceptanything/illacceptanything,gcourtemanche/rails,travisofthenorth/rails,kmcphillips/rails,BlakeWilliams/rails,shioyama/rails,kddeisz/rails,yawboakye/rails,schuetzm/rails,tgxworld/rails,bogdanvlviv/rails,yhirano55/rails,tgxworld/rails,illacceptanything/illacceptanything,untidy-hair/rails,repinel/rails,lcreid/rails,fabianoleittes/rails,schuetzm/rails,fabianoleittes/rails,odedniv/rails,Erol/rails,mechanicles/rails,assain/rails,joonyou/rails,mohitnatoo/rails,kamipo/rails,illacceptanything/illacceptanything,printercu/rails,shioyama/rails,Vasfed/rails,iainbeeston/rails,MSP-Greg/rails,illacceptanything/illacceptanything,printercu/rails,alecspopa/rails,gauravtiwari/rails,Stellenticket/rails,Envek/rails,brchristian/rails,mechanicles/rails,utilum/rails,odedniv/rails,jeremy/rails,yasslab/railsguides.jp,repinel/rails,Edouard-chin/rails,arunagw/rails,lcreid/rails,yhirano55/rails,untidy-hair/rails,kddeisz/rails,aditya-kapoor/rails,Envek/rails,mohitnatoo/rails,georgeclaghorn/rails,gfvcastro/rails,felipecvo/rails,esparta/rails,flanger001/rails,felipecvo/rails,georgeclaghorn/rails,travisofthenorth/rails,jeremy/rails,flanger001/rails,pvalena/rails,kamipo/rails,georgeclaghorn/rails,bogdanvlviv/rails,jeremy/rails,aditya-kapoor/rails,Erol/rails,jeremy/rails,utilum/rails,kamipo/rails,Vasfed/rails,deraru/rails,illacceptanything/illacceptanything,gfvcastro/rails,betesh/rails,assain/rails,betesh/rails,iainbeeston/rails,arunagw/rails,printercu/rails,Vasfed/rails,esparta/rails,starknx/rails,kmayer/rails,palkan/rails,vipulnsward/rails,utilum/rails,mohitnatoo/rails,brchristian/rails,BlakeWilliams/rails,arunagw/rails,notapatch/rails,BlakeWilliams/rails,yalab/rails,repinel/rails,gcourtemanche/rails,gauravtiwari/rails,palkan/rails,shioyama/rails,repinel/rails,gcourtemanche/rails,tjschuck/rails,palkan/rails,Envek/rails,arunagw/rails,Edouard-chin/rails,tjschuck/rails,betesh/rails,mohitnatoo/rails,yawboakye/rails,yawboakye/rails,schuetzm/rails,illacceptanything/illacceptanything,Edouard-chin/rails,eileencodes/rails,yahonda/rails,illacceptanything/illacceptanything,illacceptanything/illacceptanything,odedniv/rails,MSP-Greg/rails,fabianoleittes/rails,yalab/rails,kirs/rails-1,yahonda/rails,shioyama/rails,notapatch/rails,illacceptanything/illacceptanything,alecspopa/rails,travisofthenorth/rails,utilum/rails,gfvcastro/rails,prathamesh-sonpatki/rails,notapatch/rails,deraru/rails,rails/rails,esparta/rails,EmmaB/rails-1,illacceptanything/illacceptanything,lcreid/rails,kirs/rails-1,baerjam/rails,eileencodes/rails,vipulnsward/rails,rails/rails,flanger001/rails,Vasfed/rails,flanger001/rails,kirs/rails-1,Envek/rails,yasslab/railsguides.jp,vipulnsward/rails,illacceptanything/illacceptanything,kmcphillips/rails,MSP-Greg/rails,tgxworld/rails,betesh/rails,MSP-Greg/rails,Sen-Zhang/rails,yahonda/rails