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 ‘void rocksdb::_Test_DBIterator::_Run()’:
./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 ‘TCONCAT1’
 #define TCONCAT1(a,b) a##b
                       ^
./util/testharness.h:147:6: note: in expansion of macro ‘TCONCAT’
 void TCONCAT(_Test_,name)::_Run()
      ^
db/db_iter_test.cc:589:1: note: in expansion of macro ‘TEST’
 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