test: debug-signal-cluster should not be racey

unref one superfluous timer (as the test suite already has a global
timeout), and improve the state machine to iterate the messages more
reliably.

Ultimately make the test complete more quickly.

Original-PR-URL: [unknown]
Signed-off-by: Julien Gilli <julien.gilli@joyent.com>

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


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Bert Belder <bertbelder@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>