chore: migrate tokenserver tests to pytest with junit output

[nshirley]

Description

Describe these changes.

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The original ticket was scoped to move all python tests (tokenserver, mysqle2e and spannere2e) to utilize pytest, but for the sake of speed the latter two will be tackled in another ticket.

Interestingly, by using pytest it can discover the existing unittest tests and run them without extra work at this time, and in doing so, it's also running more tests that were not being run before! Here are the tests that additionally run:

TestProcessAccountEventsForceSpanner::test_delete_user_force_spanner
TestMigrationRecords::test_purging_no_override
TestMigrationRecords::test_purging_override_null_keys_changed_at
TestMigrationRecords::test_purging_override_with_migrated
TestMigrationRecords::test_purging_override_with_migrated_password
TestMigrationRecords::test_purging_replaced_at

Should we keep these tests in? Do they need to run?

Testing

How should reviewers test?

• Tokenserver integration tests still run, pass, and now capture results in CircleCI
  • Passing tests
  • Artifacts

Issue(s)

Closes SYNC-4516.

[Trinaa]

LGTM 🚢. Don't forget to squash before merging.

What is the ticket number for the remaining work for e2e tests? I don't see it in the epic atm.

[pjenvey]

nit: more of a note to @Trinaa and @jrconlin as we have this in autopush too -- it seems .ONESHELL is a Makefile wide special target that only needs to be set once, "If the .ONESHELL special target appears anywhere in the makefile then all recipe lines for each target.."

[pjenvey]

@nshirley and don't forget to add a "chore:" prefix to the commit when you squash