Example scenario: Run a test on Chrome OS devices that are on kernel 3.10 and higher (excluding 3.8).  Or run a test on devices with a touchpad but without touchscreen.

Currently the options for excluding certain machines are:
- Set up labels for "X" and "not X" and have tests use whichever is needed.  This solution doesn't scale in the kernel case, where having one machine with "3.10, not3.8, not3.14 ..." labels is too many.
- Let the test run on all devices and then, during the test, return TEST_NA if the DUT doesn't fit.  But we could save that time if there were a way to skip scheduling the test in the first place.