x/tools/go/analysis/analysistest: RunWithSuggestedFixes doesn't fail if all fixes are missing

[adonovan]

analysistest.RunWithSuggestedFixes loops over all the files affected by fixes and checks that each one has the expected content specified by *.golden files. However, this means that if all fixes are missing, it does no work at all. It should also check that each golden file was matched at least once.

(Adapted from Google internal issue b/414822214)