Bug 1287007 - Fix timing issue in test_delay_update_webextension.js r=rhelmer

The onUpdateAvailable listener is asynchronously notified, so the caller
(i.e. the test) cannot assume that a message sent immediately after
triggering an update would trigger the `browser.test.onMessage` listener
that was added in the onUpdateAvailable event handler.

MozReview-Commit-ID: 12n64f5l3RA

--HG--
extra : rebase_source : 6ba7923359786c4ccd7f4d29664a8567e253e0cd

toolkit/mozapps/extensions/test/xpcshell/test_delay_update_webextension.js

@@ -214,6 +214,7 @@ add_task(function* delay_updates_defer() {
             browser.test.fail(`wrong message: ${msg}`);
           }
         });
+        browser.test.sendMessage("truly ready");
       });
       browser.test.sendMessage("ready");
     },
@@ -249,6 +250,7 @@ add_task(function* delay_updates_defer() {
   do_check_eq(addon_postponed.type, "extension");
 
   // add-on will not allow upgrade until message is received
+  yield extension.awaitMessage("truly ready");
   extension.sendMessage("allow");
   yield extension.awaitFinish("allowed");