Use gnome-shell-perf-helper to control windows during perf tests

* Run gnome-shell-perf-helper during performance tests
* Use MUTTER_WM_CLASS_FILTER to omit all other windows
* Add new Scripting methods: createTestWindow,
  waitTestWindows, destroyTestWindows
* Create a single 640x480 test window for testing overview
  animation performance.

https://bugzilla.gnome.org/show_bug.cgi?id=644265

js/perf/core.js

@@ -38,6 +38,10 @@ function run() {
                               Scripting.scriptEvent('overviewShowDone');
                           });
 
+    Scripting.destroyTestWindows();
+    Scripting.createTestWindow(640, 480, false, false);
+    Scripting.waitTestWindows();
+
     yield Scripting.sleep(1000);
     yield Scripting.waitLeisure();
     for (let i = 0; i < 2; i++) {