test_result: use "MISSING" instead of "UNKNOWN"

When 0 tests are ran, for example because of an import error in the test, it appears as "UNKNOWN" and because the view of test results sort lines alphabetically, we have:

a flaky test that developer will ignore ..... FAILED
lots ........................................ PASS
of .......................................... PASS
tests ....................................... PASS
oops an error that we miss easily ........... UNKNOWN

with this change, this becomes:

a flaky test that developer will ignore ..... FAILED
oops an error that we will notice ........... MISSING
lots ........................................ PASS
of .......................................... PASS
tests ....................................... PASS