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

js/ui/environment.js

@@ -0,0 +1,32 @@
+/* -*- mode: js2; js2-basic-offset: 4; indent-tabs-mode: nil -*- */
+
+const St = imports.gi.St;
+
+const Tweener = imports.ui.tweener;
+
+// "monkey patch" in some varargs ClutterContainer methods; we need
+// to do this per-container class since there is no representation
+// of interfaces in Javascript
+function _patchContainerClass(containerClass) {
+    // This one is a straightforward mapping of the C method
+    containerClass.prototype.child_set = function(actor, props) {
+        let meta = this.get_child_meta(actor);
+        for (prop in props)
+            meta[prop] = props[prop];
+    };
+
+    // clutter_container_add() actually is a an add-many-actors
+    // method. We conveniently, but somewhat dubiously, take the
+    // this opportunity to make it do something more useful.
+    containerClass.prototype.add = function(actor, props) {
+        this.add_actor(actor);
+        if (props)
+            this.child_set(actor, props);
+    };
+}
+
+_patchContainerClass(St.BoxLayout);
+
+function init() {
+    Tweener.init();
+}