Remove some unreliable usages of setTimeout( ,0) from layout tests

LayoutTests/http/tests/images/png-partial-load-no-alpha.html

 <img style="background: green" >
 <script>
 if (window.testRunner) {
     testRunner.dumpAsTextWithPixelResults();
     testRunner.waitUntilDone();
 }
 
 function loadAndStall()
 {
     return "http://127.0.0.1:8000/resources/load-and-stall.php";
 }
 
 function pngImage()
 {
     return "?name=../../../fast/images/resources/lenna.png&mimeType=image%2Fpng";
 }
 
 function testDone()
 {
     if (window.testRunner)
         testRunner.notifyDone();
 }
 
 function runTest()
 {
     document.querySelector("img").src = loadAndStall() + pngImage() + "&stallAt=55297&stallFor=60";
     setTimeout(testDone, 500);
 }
 
-setTimeout(runTest, 0);
+window.onload = function() {
+    setTimeout(runTest, 0);
+}
 </script>