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

Ref gh-3040
Closes gh-3049

test/data/offset/fixed.html

@@ -13,6 +13,7 @@
 			#marker { position: absolute; border: 2px solid #000; width: 50px; height: 50px; background: #ccc; }
 		</style>
 		<script src="../../jquery.js"></script>
+		<script src="../iframeTest.js"></script>
 		<script type="text/javascript" charset="utf-8">
 			jQuery(function($) {
 				window.scrollTo(1000,1000);
@@ -20,7 +21,7 @@
 					$("#marker").css( $(this).offset() );
 					return false;
 				});
-				window.parent.iframeCallback( jQuery, window, document );
+				startIframeTest();
 			});
 		</script>
 	</head>