fix(issue-owners): Fix problem where issue owner order is not changed on new events

[malwilley]

WIP

This just adds the tests which I used to verify that this was the underlying problem. Passing the work off to the next triage rotation.

Also note that this issue only seems to apply for teams and not users, since the logic here checks for User instead of RpcUser when looking for existing owners.

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
21498	1	21497	205

View the top 1 failed tests by shortest run time

tests.sentry.tasks.test_post_process.PostProcessGroupErrorTest test_owner_assignment_existing_owners_different_order

Stack Traces | 8.13s run time

#x1B[1m#x1B[.../sentry/tasks/test_post_process.py#x1B[0m:996: in test_owner_assignment_existing_owners_different_order
    assert assignee.team == new_assigned_team
#x1B[1m#x1B[31mE   AssertionError: assert <Team at 0x7f5a54b3cd10: id=4554671932702721, name='foo', slug='foo'> == <Team at 0x7f5a55552120: id=4554671932768256, name='Awake Honeybee', slug='awake-honeybee'>#x1B[0m
#x1B[1m#x1B[31mE    +  where <Team at 0x7f5a54b3cd10: id=4554671932702721, name='foo', slug='foo'> = <GroupAssignee at 0x7f5a54bdad50: id=36, group_id=632, user_id=None, team_id=4554671932702721>.team#x1B[0m

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