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/expected/all-window-properties.txt

@@ -395,6 +395,7 @@ asyncTest
 printElement
 println
 promiseTest
+removeTestErrorHandler
 spoofCurrentURL
 test
 timeout

Tests/LibWeb/Text/input/HTML/Window-onerror.html

@@ -2,6 +2,9 @@
 <script src="../include.js"></script>
 <script>
     asyncTest(done => {
+
+        removeTestErrorHandler()
+
         window.addEventListener("error", (event) => {
             println(`onerror event fired: ${event.error}`);
             done();

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}`);

Tests/LibWeb/Text/input/include.js

@@ -52,6 +52,20 @@ function timeout(ms) {
     return promise;
 }
 
+const __testErrorHandlerController = new AbortController();
+window.addEventListener(
+    "error",
+    event => {
+        println(`Uncaught Error In Test: ${event.message}`);
+        __finishTest();
+    },
+    { signal: __testErrorHandlerController.signal }
+);
+
+function removeTestErrorHandler() {
+    __testErrorHandlerController.abort();
+}
+
 document.addEventListener("DOMContentLoaded", function () {
     __outputElement = document.createElement("pre");
     __outputElement.setAttribute("id", "out");