run the single test file specified in the args param

This change makes it possible to run the tests in a single file by specifying its class name in the *args* URL parameter, i.e. to run just the *TestInterIsolateMutex* tests, you would specify `args=com/sun/midp/util/isolate/TestInterIsolateMutex` in the URL params.
Yeah I was wondering why you weren't just looping over Testlets.list and using String::equals, doesn't matter anyway ;)