It simply does nothing. This is because skipping the entire suite was added to skip tests when they can't be run because of the environment, for example to skip the a11y tests when the at-spi daemons fail to start. However, some tests have been skipped entirely lately because they fail or timeout under xvfb, but the script now allows to run tests without xvfb. I think we should disable tests that are impossible to run because of the environment instead of skipping them, and make skip options work with all skipped tests.
Created attachment 211408 [details] Patch
Committed r155605: <http://trac.webkit.org/changeset/155605>