Fix "warning: instance variable @defined_root not initialized"
Fix "warning: instance variable @defined_root not initialized"

Currently, the following error is shows only when run the test using
`bin/test`.

```
./bin/test -w test/template/log_subscriber_test.rb
Run options: --seed 17167

# Running:

/rails/actionview/test/template/log_subscriber_test.rb:34: warning: instance variable @defined_root not initialized
```

In `AVLogSubscriberTest`, if the `Rails.root` is not defined, define the
method and undef it in teardown.
https://github.com/rails/rails/blob/master/actionview/test/template/log_subscriber_test.rb#L21..L33

However, in `bin/test`, `Rails.root` is defined, which results in referring to
uninitialized variables and warnings.

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