Use script instead of fetch to de-flake firefox

preload/prefetch-cache.html

@@ -8,19 +8,22 @@
 <script>
 
 async function prefetch_and_count(cacheControl, t) {
-    const {href} = await prefetch({"cache-control": cacheControl, "type": "text/plain", content: "123"}, t);
-    await fetch(href);
+    const {href} = await prefetch({
+        "cache-control": cacheControl,
+        "type": "application/javascript",
+        content: "/**/"}, t);
+    const script = document.createElement("script");
+    script.src = href;
+    t.add_cleanup(() => script.remove());
+    const loaded = new Promise(resolve => script.addEventListener("load", resolve));
+    document.body.appendChild(script);
+    await loaded;
     const info = await get_prefetch_info(href);
     return info.length;
 }
 
 promise_test(async t => {
-    let result = 0;
-    // Sometimes the HTTP cache is not reliable. We want to see that the prefetch is reused in at
-    // least 1 out of 10 tries.
-    for (let i = 0; i < 10 && result !== 1; ++i) {
-        result = await prefetch_and_count("max-age=604800", t);
-    }
+    const result = await prefetch_and_count("max-age=604800", t);
     assert_equals(result, 1);
 }, "Prefetch should populate the HTTP cache");