Bug 1451513 Part 3: Make loading the mochitest extension more robust r=kmag

A couple of changes to ensure that the mochitest harness doesn't try to
start executing changes before the mochitest extension is loaded:
1. Fix the marionette driver to wait for an installed extension to
   be started before returning from Addon:install
2. Wait for extension API onStartup() handlers to finish before
   considering a webextension started.

MozReview-Commit-ID: 8YEdNn6s5qh

--HG--
extra : rebase_source : 67e9abadcda82d55ac73c33367ec65cdbf7b823d

testing/marionette/addon.js

@@ -23,37 +23,12 @@ const ERRORS = {
 async function installAddon(file) {
   let install = await AddonManager.getInstallForFile(file);
 
-  return new Promise(resolve => {
   if (install.error) {
     throw new UnknownError(ERRORS[install.error]);
   }
 
-    let addonId = install.addon.id;
-
-    let success = install => {
-      if (install.addon.id === addonId) {
-        install.removeListener(listener);
-        resolve(install.addon);
-      }
-    };
-
-    let fail = install => {
-      if (install.addon.id === addonId) {
-        install.removeListener(listener);
+  return install.install().catch(err => {
     throw new UnknownError(ERRORS[install.error]);
-      }
-    };
-
-    let listener = {
-      onDownloadCancelled: fail,
-      onDownloadFailed: fail,
-      onInstallCancelled: fail,
-      onInstallFailed: fail,
-      onInstallEnded: success,
-    };
-
-    install.addListener(listener);
-    install.install();
   });
 }
 

toolkit/components/extensions/Extension.jsm

@@ -1778,9 +1778,11 @@ class Extension extends ExtensionData {
       // and it is used to run code that needs to be executed before
       // any of the "startup" listeners.
       this.emit("startup", this);
-      Management.emit("startup", this);
 
-      await this.runManifest(this.manifest);
+      await Promise.all([
+        Management.emit("startup", this),
+        this.runManifest(this.manifest),
+      ]);
 
       Management.emit("ready", this);
       this.emit("ready");