Bug 1635806 - [testing] Expose eventName in BrowserTestUtils.waitForContentEvent r=ahal

This should make it easier to write tests for race-conditions, as
the resolving value now indicates which event was received.

This required updating a test which for some reason asserted
undefined being the resolving value, as a way of "testing" if the
Promise resolved at all. Since await already throws Errors when it
is rejected, and the test throws a timeout if it doesn't resolve in
time, this assertion was removed. It did not add any value.

Differential Revision: https://phabricator.services.mozilla.com/D74124

dom/html/test/browser_fullscreen-contextmenu-esc.js

@@ -106,8 +106,7 @@ add_task(async function() {
     "fullscreenchange"
   );
   EventUtils.synthesizeKey("KEY_Escape");
-  state = await fullscreenExitPromise;
-  ok(!state, "The content should have exited fullscreen");
+  await fullscreenExitPromise;
   ok(!document.fullscreenElement, "The chrome should have exited fullscreen");
 
   gBrowser.removeTab(tab);

testing/mochitest/BrowserTestUtils/BrowserTestUtils.jsm

@@ -1193,7 +1193,7 @@ var BrowserTestUtils = {
         eventName,
         () => {
           removeEventListener();
-          resolve();
+          resolve(eventName);
         },
         { capture, wantUntrusted },
         checkFn