Tests: Make iframe tests wait after checking isReady

Ref gh-3040

test/data/selector/sizzle_cache.html

@@ -17,5 +17,10 @@
 	<div class="test">
 		<a href="#" id="collision">Worlds collide</a>
 	</div>
+	<script>
+		jQuery( function() {
+			window.parent.iframeCallback( jQuery, window, document );
+		} );
+	</script>
 </body>
 </html>