Bug 905437: Make it possible for 'mach xpcshell-test' to run xpcshell under GDB. r=ted

At the moment, the --debug argument accepted by mach xpcshell-test is
dropped. I don't know if just bringing the old harness's command-line
arguments forward to 'mach xpcshell-test' is the most mach-ish way to do
things, but it does let me get things started.