Bug 972093 - Detect run_next_test() during add_task(); r=ted, rnewman

If an xpcshell test called run_next_test() from inside an add_task(),
bad things would happen. This patch detects that behavior and aborts the
test immediately with an actionable error message.

--HG--
extra : rebase_source : c5cf11070421b95a6fcf476d0a86289a3d0ba73a
extra : amend_source : cec6ad2a751bba01e7d4ecbff0735b39e3326efb