In two cases, we check for the content of an error right after asserting that
no error occurs. Fix the sense of those checks.
In one case, we check that there is no error with the diagnostic "expected
error". It's not clear whether this means "an error was expected" (which is
what I believe) or "we got the expected error". However, given the way the mock
plumbing is set up, the first interpretation seems right.