Summary:
After af92d4ad, only exclusive manual compaction can have conflict. dc360df8 updated the conflict-checking test case accordingly. But we missed the point that exclusive manual compaction can only conflict with automatic compactions scheduled after it, since it waits on pending automatic compactions before it begins running.

This PR updates the test case to ensure the automatic compactions are scheduled after the manual compaction starts but before it finishes, thus ensuring a conflict. I also cleaned up the test case to use less space as I saw it cause out-of-space error on travis.
Closes https://github.com/facebook/rocksdb/pull/3375

Differential Revision: D6735162

Pulled By: ajkr

fbshipit-source-id: 020530a4e150a4786792dce7cec5d66b420cb884