Users are sometimes confused why their test doesn't run. It is usually
because of a misspelled testname, for example using 'TEST=1234' instead
of 'TEST=T1234'. After this patch it is hopefully more clear what the
problem is, showing:

    ERROR: tests not found: ['1234']

Instead of:

    0 total tests, which gave rise to
    0 test cases, of which
    0 were skipped

Reviewed by: austin, bgamari

Differential Revision: https://phabricator.haskell.org/D1388