Summary:
Add a unit test case to check memory usage when
max_write_buffer_size_to_maintain is set if flushed immutable memtables are
trimmed timely or not.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7311

Test Plan: Compared the results with before bug fix.

Reviewed By: ltamasi

Differential Revision: D23321702

Pulled By: akankshamahajan15

fbshipit-source-id: da04ee21137d641a07fd499a9e2749eb036fcb1e