testbot: Allow looking up the bug corresponding to a given test failure.

This can be used on any test report, whether it is from test.winehq.org or running a test manually, and consolidates the bug reports as the place to brainstorm and get more information on test failures.

This is also very practical when one wants to check whether a given GitLab CI test failure is known to the TestBot (if it is then it is not caused by the specific merge request).