Add a test that forces a crash. This makes it easy to manually check the output of a segfault with a complicated backtrace; the actual automation doesn't help us except to verify that the crash continues to happen as excepted.

components/script/dom/webidls/TestBinding.webidl

@@ -456,3 +456,8 @@ interface TestBinding {
   [Func="TestBinding::condition_satisfied"]
   const unsigned short funcControlledConstEnabled = 0;
 };
+
+partial interface TestBinding {
+  [Pref="dom.testable_crash.enabled"]
+  void crashHard();
+};