test: fix timing issue in signal test

Change sequential/test-signal-unregister so it doesn't use fixed
timeouts for sending the signal and expecting the child to quit.

Fixes: https://github.com/iojs/io.js/issues/1223
PR-URL: https://github.com/iojs/io.js/pull/1227


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>