Improve the mochitest to wait for the button to appear before startin… (

#2759)

* Improve the mochitest to wait for the button to appear before starting. Also use the mochitest config from the mozilla plugin to save specifying globals.

* Correctly enable the mozilla plugin for ESLint

.eslintrc.yml

@@ -14,6 +14,7 @@ parserOptions:
     jsx: true
 
 plugins:
+  - mozilla
   - react
   - promise
 

test/addon/.eslintrc.js

@@ -0,0 +1,7 @@
+"use strict";
+
+module.exports = {
+  "extends": [
+    "plugin:mozilla/browser-test"
+  ]
+}

test/addon/browser_screenshots_ui_check.js

@@ -1,8 +1,5 @@
 "use strict";
 
-/* global add_task, is, promiseScreenshotsEnabled, promiseScreenshotsReset,
-   registerCleanupFunction */
-
 function checkElements(expectPresent, l) {
   for (let id of l) {
     is(!!document.getElementById(id), expectPresent, "element " + id + (expectPresent ? " is" : " is not") + " present");
@@ -16,5 +13,9 @@ add_task(function*() {
     yield promiseScreenshotsReset();
   });
 
+  yield BrowserTestUtils.waitForCondition(
+    () => document.getElementById("screenshots_mozilla_org-browser-action"),
+    "Screenshots button should be present", 100, 100);
+
   checkElements(true, ["screenshots_mozilla_org-browser-action"]);
 });