Tests: Refactor testIframe() to make it DRYer and more consistent

Ref gh-3040
Closes gh-3049

test/data/iframeTest.js

@@ -0,0 +1,8 @@
+
+window.startIframeTest = function() {
+	var args = Array.prototype.slice.call( arguments );
+
+	// Note: jQuery may be undefined if page did not load it
+	args.unshift( window.jQuery, window, document );
+	window.parent.iframeCallback.apply( null, args );
+};