script/ptl-tool: add unit tests for the credential fail-fast fix

Extract the Redmine auth check added in the previous commit into a
standalone verify_redmine_auth(R) function so it can be unit tested
in isolation, and add src/script/test_ptl_tool.py covering:

- verify_redmine_auth(): invalid key (AuthError), a key that
  authenticates but lacks permission (ForbiddenError), a valid key,
  and confirming unrelated Redmine errors (ServerError) still
  propagate normally rather than being misreported as a credentials
  problem.
- AuditReport.post_consolidated_review(): logs success on 2xx, logs
  an error (rather than failing silently) on a non-2xx GitHub
  response, never calls out to GitHub under --dry-run, and is a
  no-op when the report has no issues to post.

No behavior change to verify_redmine_auth() itself -- this is a pure
extraction plus tests. ptl-tool.py has no existing test suite, so
these are self-contained (mocked Redmine/requests, no network access,
no real git checkout needed) and run with plain pytest; see the
module docstring for the exact invocation.

Signed-off-by: Yuri Weinstein <yweinste@redhat.com>