python/tests: logging: use configure api for log env (#1932)

Summary:
Pull Request resolved: #1932

replace env-var overrides with `configure`. relax `test_flush_logs_ipython` which before was over-constrained and consequently, highly flaky.

Reviewed By: mariusae

Differential Revision: D87449483

fbshipit-source-id: 5c22e19878c1c034ff2ed8e3dc0c0c6d683ec208

Author: shayne-fletcher