We have two tests that use FileCheck for diagnostics and which try to
check that the output contains no compiler errors by checking for the
string 'error'. The issue with this approach is that this also causes
those tests to fail if the *path* contains the word 'error', which can
happen e.g. if the branch name contains the word 'error'.
Instead, we now check for `error:` since that string is much less likely
to appear in a path.
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