Using malloc_usable_size() in arena triggers compactions early and results in smaller files and diff number of files

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### Expected behavior

Some tests such as column_family_test,db_compaction_test rely on producing a specific number of SST files in directories. To that end they employ a `SpecialMemTable` rep that causes flushes to take place once once a certain threshold is reached.

### Actual behavior

However, the compaction is triggered based on the amount of memory used within the `Memtable`. Some debug and non-debug allocators have a higher allocation overhead and the `malloc_usable_size` reported is higher. This results in earlier flushes, smaller files, smaller sizes per level or db path.

Some tests such as `db_compaction_test` and `column_family_test` expect certain number of files to be created as a result of inserting data. This creates a erroneous connection between the amount data inserted and the amount of memory allocated within the `Memtable`, and this results in smaller amount of data inserted per file, the number of files is higher and the tests fail.

IMHO, either the number

### Steps to reproduce the behavior