LibWeb: Convert some sync tests to be async

The events tested here are decidedly async. We also can't really write
sync tests of the form "test(async () => {})". Nothing will await the
async callback.

Tests/LibWeb/Text/input/link-element-rel-preload-load-event.html

@@ -1,12 +1,18 @@
 <script src="include.js"></script>
 <script>
-    test(() => {
+    asyncTest(done => {
+        let eventCount = 0;
+
         let goodLink = document.createElement("link");
         goodLink.setAttribute("rel", "preload");
         goodLink.setAttribute("href", "valid.css");
         goodLink.setAttribute("as", "style");
         goodLink.addEventListener("load", function () {
             println("Got load event");
+
+            if (++eventCount == 2) {
+                done();
+            }
         });
         document.head.appendChild(goodLink);
 
@@ -16,6 +22,10 @@
         badLink.setAttribute("as", "style");
         badLink.addEventListener("error", function () {
             println("Got error event");
+
+            if (++eventCount == 2) {
+                done();
+            }
         });
         document.head.appendChild(badLink);
     });