Add some structure for interactive tests of UI components

js/ui/environment.js: Split out initial UI setup (Tweener initialization,
  ClutterContainer monkey-patching) into a separate file we can import from tests.

tests/: Directory for various types of tests
tests/run-test.sh: Shell script that to run tests with an appropriate
  environment set up.

tests/testcommon/: Common modules and data for tests
tests/interactive/: Interactive tests

tests/interactive/box-layout.js: A sample test of StLayout

https://bugzilla.gnome.org/show_bug.cgi?id=595987

tests/testcommon/test.css

@@ -0,0 +1,13 @@
+@import "../../data/gnome-shell.css";
+
+*.red {
+    background-color: red;
+}
+
+*.green {
+    background-color: green;
+}
+
+*.blue {
+    background-color: blue;
+}

tests/testcommon/ui.js

@@ -0,0 +1,16 @@
+/* -*- mode: js2; js2-basic-offset: 4; indent-tabs-mode: nil -*- */
+
+const Clutter = imports.gi.Clutter;
+const GLib = imports.gi.GLib;
+const St = imports.gi.St;
+
+const Environment = imports.ui.environment;
+
+function init() {
+    Clutter.init(null, null);
+    Environment.init();
+
+    let style = St.Style.get_default();
+    let stylesheetPath = GLib.getenv("GNOME_SHELL_TESTSDIR") + "/testcommon/test.css";
+    style.load_from_file(stylesheetPath);
+}