[test] re-enable t/pmc/signal.t
[test] Re-enabled instable signals tests as TODO

signals do work fine manually, just the tests not.
eg. on darwin:
$ (./parrot t/pmc/signal_1.pasm &); ps | grep "./[p]arrot" | cut -c1-6 | xargs kill -HUP
start
$ (./parrot t/pmc/signal_3.pasm &); ps | grep "./[p]arrot" | cut -c1-6 | xargs kill -HUP
start
Merge branch 'smoke-me/signal-tests'