CI trial runs annotated with Signatures do not match

[DMRobertson]

1. GHA invokes trial which runs the test tests.handlers.test_oidc.OidcHandlerTestCase.test_callback_session.
2. This encounters debug logging added in Add debug for OIDC flow #9307. This uses logger.exception which writes a traceback to synapse's log. It will eventually appear in the test output when trial summaries the test run. (Unclear if this is on stdout or stderr)
3. We use actions/setup-python which sets up a problem matcher that looks for tracebacks ending in a raise statement containing a string literal.
4. The (intentionally-logged) tracebacks end up getting flagged as a scary annotation. But this is a false positive.

Suggestions:

• Remove the logger.exception if we don't need it any more
• Log something else that's useful but which won't trigger the problem matcher
• Setup test logging config to suppress logging.exceptions in this module

(Surprised this hasn't caught annotated more exceptions in the logs tbh---perhaps the regex only detects single-quoted string literal exception messages?)