act: Add test for bypassing queueMicrotask (#21743)

Test for fix added in #21740

packages/react-reconciler/src/__tests__/ReactIsomorphicAct-test.js

@@ -0,0 +1,50 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @jest-environment node
+ */
+
+// sanity tests for act()
+
+let React;
+let ReactNoop;
+let act;
+let DiscreteEventPriority;
+
+describe('isomorphic act()', () => {
+  beforeEach(() => {
+    React = require('react');
+    ReactNoop = require('react-noop-renderer');
+    DiscreteEventPriority = require('react-reconciler/constants')
+      .DiscreteEventPriority;
+    act = React.unstable_act;
+  });
+
+  // @gate __DEV__
+  test('bypasses queueMicrotask', async () => {
+    const root = ReactNoop.createRoot();
+
+    // First test what happens without wrapping in act. This update would
+    // normally be queued in a microtask.
+    ReactNoop.unstable_runWithPriority(DiscreteEventPriority, () => {
+      root.render('A');
+    });
+    // Nothing has rendered yet
+    expect(root).toMatchRenderedOutput(null);
+    // Flush the microtasks by awaiting
+    await null;
+    expect(root).toMatchRenderedOutput('A');
+
+    // Now do the same thing but wrap the update with `act`. No
+    // `await` necessary.
+    act(() => {
+      ReactNoop.unstable_runWithPriority(DiscreteEventPriority, () => {
+        root.render('B');
+      });
+    });
+    expect(root).toMatchRenderedOutput('B');
+  });
+});