LibWeb: Push a temporary execution context for setTimeout

This fixes a crash seen running stream tests.

Tests/LibWeb/Text/input/wpt-import/streams/resources/test-utils.js

@@ -0,0 +1,27 @@
+'use strict';
+
+self.delay = ms => new Promise(resolve => step_timeout(resolve, ms));
+
+// For tests which verify that the implementation doesn't do something it shouldn't, it's better not to use a
+// timeout. Instead, assume that any reasonable implementation is going to finish work after 2 times around the event
+// loop, and use flushAsyncEvents().then(() => assert_array_equals(...));
+// Some tests include promise resolutions which may mean the test code takes a couple of event loop visits itself. So go
+// around an extra 2 times to avoid complicating those tests.
+self.flushAsyncEvents = () => delay(0).then(() => delay(0)).then(() => delay(0)).then(() => delay(0));
+
+self.assert_typed_array_equals = (actual, expected, message) => {
+  const prefix = message === undefined ? '' : `${message} `;
+  assert_equals(typeof actual, 'object', `${prefix}type is object`);
+  assert_equals(actual.constructor, expected.constructor, `${prefix}constructor`);
+  assert_equals(actual.byteOffset, expected.byteOffset, `${prefix}byteOffset`);
+  assert_equals(actual.byteLength, expected.byteLength, `${prefix}byteLength`);
+  assert_equals(actual.buffer.byteLength, expected.buffer.byteLength, `${prefix}buffer.byteLength`);
+  assert_array_equals([...actual], [...expected], `${prefix}contents`);
+  assert_array_equals([...new Uint8Array(actual.buffer)], [...new Uint8Array(expected.buffer)], `${prefix}buffer contents`);
+};
+
+self.makePromiseAndResolveFunc = () => {
+  let resolve;
+  const promise = new Promise(r => { resolve = r; });
+  return [promise, resolve];
+};