Document Atest support for wildcards in ART run-tests invocations.

Mention that Atest supports wildcards in test names and how this can
be used to run ART run-tests in `art/test/README.atest.md`.

Test: Render `README.atest.md` as HTML and check the output manually
Bug: 147818031
Change-Id: I0b9771a4b9685b2cb27cd95a0feeba1d22ff430b
1 file changed