Summary:
Add `experiemental_allow_mempurge` flag support for `db_stress` and `db_crashtest.py`, with a `false` default value.
I succesfully tested locally both `whitebox` and `blackbox` crash tests with `experiemental_allow_mempurge` flag set as true.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8545

Reviewed By: akankshamahajan15

Differential Revision: D29734513

Pulled By: bjlemaire

fbshipit-source-id: 24316c0eccf6caf409e95c035f31d822c66714ae