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.
Nit: Why creating a vector using the Testlets.list values instead of directly checking them?