Divide test DBIteratorTest.DBIterator to smaller tests
Divide test DBIteratorTest.DBIterator to smaller tests

Summary:
When building on my host, I saw warning:

In file included from db/db_iter_test.cc:17:0:
db/db_iter_test.cc: In member function â€˜void rocksdb::_Test_DBIterator::_Run()â€™:
./util/testharness.h:147:14: note: variable tracking size limit exceeded with -fvar-tracking-assignments, retrying without
 void TCONCAT(_Test_,name)::_Run()
              ^
./util/testharness.h:134:23: note: in definition of macro â€˜TCONCAT1â€™
 #define TCONCAT1(a,b) a##b
                       ^
./util/testharness.h:147:6: note: in expansion of macro â€˜TCONCATâ€™
 void TCONCAT(_Test_,name)::_Run()
      ^
db/db_iter_test.cc:589:1: note: in expansion of macro â€˜TESTâ€™
 TEST(DBIteratorTest, DBIterator) {
 ^

By dividing the test into small tests, it should fix the problem

Test Plan: Run the test

Reviewers: rven, yhchiang, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D32679

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