test: move sequential/test-debug-port-from-cmdline

Move sequential/test-debug-port-from-cmdline to test/parallel.  Per the
previous commit, it should now be possible to run the test in parallel
with other debugger tests.

PR-URL: https://github.com/iojs/io.js/pull/501


Reviewed-By: Bert Belder <bertbelder@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>