ref: fix test pollution integrity errors due to cache leaks between tests

[asottile-sentry]

this results in about a 15% failure rate:

$ pytest --count 10 tests/sentry/event_manager/test_event_manager_grouping.py -k test_records_hash_comparison_metric
...

failing with:

E   django.db.utils.IntegrityError: ForeignKeyViolation('insert or update on table "sentry_environmentproject" violates foreign key constraint "sentry_environmentpr_environment_id_f5d02227_fk_sentry_en"\nDETAIL:  Key (environment_id)=(18) is not present in table "sentry_environment".\n')
E   SQL: SET CONSTRAINTS ALL IMMEDIATE

I traced this down to a cache leaking from one test to another -- after applying this patch I had no failures in 100x runs of the tests

[asottile-sentry]

also: the reason this is flaky is that sometimes the ids collide and sometimes they do not due to something non-deterministic in event classification -- haven't quite figured that part out yet

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #77476      +/-   ##
==========================================
- Coverage   78.11%   78.11%   -0.01%     
==========================================
  Files        6957     6957              
  Lines      308809   308809              
  Branches    50539    50539              
==========================================
- Hits       241231   241226       -5     
  Misses      61188    61188              
- Partials     6390     6395       +5