Currently, regression tests allow you to match on a single diagnostic in a file without specifying its message. This can lead to us matching the wrong diagnostic or applying the wrong quick fixes.

Whenever we wait for diagnostics, we should wait for all of the diagnostics in the file to ensure we're not producing duplicates or new diagnostics. We should also always require the diagnostic message in all cases.

Finally, we should require a single diagnostic when we apply quick fixes, instead of applying all of the quick fixes available.

/cc @heschik