Bug 254434

The MotionMark driver harness should allow users to specify a subtest or a list of multiple subtests. Currently the debug runner (developer.html) has the ability to run subtests, but: a) only one at a time, and b) only while all other options are specified (e.g, `test-interval`, `display`, etc). We should be able to have the ability to run subtests from the main MotionMark harness (index.html) using default run and iteration settings. Because only the `MotionMark` suite is available in the default harness, we may be able to get away with specifying just test name and omitting suite like we do with JetStream2 (and even if not, test names appear to be unique anyways according to the debug runner).