Bug 1586393, part 1 - Add test-complete observer notification. r=mconley

This allows generic code, like a testing JSM, to do clean up when a
test completes, in a way that will work in different test suites.

Differential Revision: https://phabricator.services.mozilla.com/D50648

--HG--
extra : moz-landing-system : lando

testing/mochitest/browser-test.js

@@ -589,6 +589,8 @@ Tester.prototype = {
         }
       }
 
+      Services.obs.notifyObservers(null, "test-complete");
+
       if (
         this.currentTest.passCount === 0 &&
         this.currentTest.failCount === 0 &&

testing/mochitest/tests/SimpleTest/SimpleTest.js

@@ -1249,6 +1249,8 @@ SimpleTest.finish = function() {
     };
 
     executeCleanupFunction();
+
+    SpecialPowers.notifyObservers(null, "test-complete");
 };
 
 /**

testing/xpcshell/head.js

@@ -647,6 +647,8 @@ function _execute_test() {
     .then(() => (complete = true));
   _Services.tm.spinEventLoopUntil(() => complete);
 
+  _Services.obs.notifyObservers(null, "test-complete");
+
   // Restore idle service to avoid leaks.
   _fakeIdleService.deactivate();