MAYBE fix flaky TestPolicySetsCreate/with_vcs_policy_updated

[nfagerlund]

Description

This flake has been extremely rowdy lately. The flake signature is:

• PoliciesPath is zeroed out
• After logging that failure, it segfaults on the next line (nil pointer dereference)

It turns out PoliciesPath is kind of a red herring — the backend erases that
value if the VCSRepo is removed, as documented in the API reference. The real
action is in VCSRepo, which comes back as a nil pointer after that update. And,
if you modify the test to do an additional Read on the policy set to compare, it
agrees with what the Update returned — the repo got nilled out for some reason.

I went looking in the backend itself, and it turns out that OAuthClients have
some async cleanup behavior on deletion. And the prior subtest that sets up the
policy set used by the flaky test cleans up its OAuthClient when it's done,
leaving the next subtest to create a new one. My working theory is that there's
a race in the backend: if the Update call with the new VCSRepo values manage to
slip in before the async cleanup for the old OAuthClient is done, it'll get
nilled out instead of creating a new VCSRepo model.

IF this guess is right, then we can avoid the flake by not trying to eagerly
clean up the first OAuthClient before the rest of the subtests run. We're
cleaning up the whole org at the end of the outer test block anyway, so there's
no real benefit to being in a hurry, and an org can have multiple OAuthClient
instances at once.

Testing plan

1. Dang test stops flaking in CI!!!!

External links

• API documentation (see the note about policies-path only being valid with vcs)

Jira is TF-9569.

Output from tests

see CI

[nfagerlund]

I've re-run this test suite like three times and can't get it to flake, whereas it was flaking every other time when I was trying to do anything ELSE in this repo this week.

[github-actions]

Reminder to the contributor that merged this PR: if your changes have added important functionality or fixed a relevant bug, open a follow-up PR to update CHANGELOG.md with a note on your changes.