If marge doesn't find deqp-runner results or trace errors, she will parse
a failed job log for error messages. The parsing ignores certain phrases
in an attempt to find more relevant errors.
Add a test to check marge's error log parsing to make sure that we don't
create regressions when adding new phrases to ignore.
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32773>