Some basic tests for MessagePanel

Check that it puts the read marker in the right place, and check that the
animation works.

... all of which has been surprisingly painful.

test/components/stub-component.js

@@ -4,8 +4,17 @@
 
 var React = require('react');
 
-module.exports = React.createClass({
-    render: function() {
-        return <div />;
-    },
-});
+module.exports = function(opts) {
+    opts = opts || {};
+    if (!opts.displayName) {
+        opts.displayName = 'StubComponent';
+    }
+
+    if (!opts.render) {
+        opts.render = function() {
+            return <div>{this.displayName}</div>;
+        }
+    }
+
+    return React.createClass(opts);
+};