LibWeb: Add a feature to LibWeb tests to fail on unhandled exceptions

Previously, if there was an unhandled exception in an async test, it
might fail to call done() and timeout. Now we have a default "error"
handler to catch unhandled exceptions and fail the test. A few tests
want to actually test the behavior of window.onerror, so they need an
escape hatch.

Tests/LibWeb/Text/input/HTML/WindowOrWorkerGlobalScope-reportError.html

@@ -1,6 +1,8 @@
 <script src="../include.js"></script>
 <script>
     test(() => {
+        removeTestErrorHandler()
+
         window.onerror = (message, filename, lineno, colno, error) => {
             println(`message = ${message}`);
             println(`lineno = ${lineno}`);