Add ability to skip run-tests with ART_TEST_RUN_TEST_SKIP.

One can skip running specific run-tests under any configuration by
setting the ART_TEST_RUN_TEST_SKIP variable to a list of the run-test
names to skip.

    $ # None of the variants of 022-interfaces will be run.
    $ ART_TEST_RUN_TEST_SKIP=022-interface mm test-art-host-run-test

Change-Id: I33f24dd7f1eb9f4d6dc407a4abc3c5919437d4fa