Remove testharness from reftest

I somehow made this both a testharness test and a reftest. This
CL converts this to just a reftest, to avoid breaking things [1].

[1] #43303

Bug: 1236777
Change-Id: I89fc76e68a24ab12c4a9efeaac36183dfb85b746
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5055427
Commit-Queue: Mason Freed <masonf@chromium.org>
Commit-Queue: Joey Arhar <jarhar@chromium.org>
Auto-Submit: Mason Freed <masonf@chromium.org>
Reviewed-by: Joey Arhar <jarhar@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1228158}

css/cssom/CSSStyleSheet-constructable-concat.html

@@ -4,14 +4,20 @@
 <link rel="author" href="mailto:masonf@chromium.org">
 <link rel="help" href="https://drafts.csswg.org/cssom/#extensions-to-the-document-or-shadow-root-interface">
 <link rel="match" href="CSSStyleSheet-constructable-concat-ref.html">
-<script src="/resources/testharness.js"></script>
-<script src="/resources/testharnessreport.js"></script>
 
 <span>This should be green</span>
 <div id=host></div>
+<pre id=errors></pre>
 
 <script>
-test(function() {
+function assert_equals(val1,val2) {
+  if (val1 === val2) {
+    return;
+  }
+  const t = document.createTextNode(`FAIL! ${val1} should be equal to ${val2}\n`);
+  errors.appendChild(t);
+}
+window.onload = () => {
   const sheet = new CSSStyleSheet();
   sheet.replaceSync('span {background-color:green;}');
   assert_equals(document.adoptedStyleSheets.length,0);
@@ -24,5 +30,5 @@
   assert_equals(shadow.adoptedStyleSheets.length,0);
   shadow.adoptedStyleSheets = shadow.adoptedStyleSheets.concat([sheet]);
   assert_equals(shadow.adoptedStyleSheets.length,1);
-}, "adoptedStyleSheets should allow .concat on empty starting values");
+};
 </script>