webkitpy: Layout tests which have no output will succeed

webkitpy: Layout tests which have no output will succeed
https://bugs.webkit.org/show_bug.cgi?id=172322
<rdar://problem/32258350>

Reviewed by David Kilzer.

* Scripts/webkitpy/layout_tests/controllers/single_test_runner.py:
(SingleTestRunner._compare_text): Add FailreNotTested to failures if test has no output.
* Scripts/webkitpy/layout_tests/models/test_failures.py:
(determine_result_type): FailureNotTested means that the test could not be loaded. This
is classified as a MISSING failure.
(FailureNotTested): Added.
(FailureNotTested.message):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@217130 268f45cc-cd09-0410-ab3c-d52691b4dbfc