fix: define IS_ACT_ENVIRONMENT global for tests with concurrent mode and synchronous act

[hoxyq]

Unblocks #28152 for landing.

While triaging why #28152 breaks some tests for DevTools, I've observed that using act from internal-test-utils is actually fixes the issue. One of the main differences between act from react and act from internal-test-utils is that later awaits for microtasks:

react/packages/internal-test-utils/internalAct.js

Lines 60 to 64 in d27c1ac

	// Call the provided scope function after an async gap. This is an extra
	// precaution to ensure that our tests do not accidentally rely on the act
	// scope adding work to the queue synchronously. We don't do this in the
	// public version of `act`, though we maybe should in the future.
	await waitForMicrotasks();

All tests that were failing with modern strict mode enabled, using concurrent mode, but with a synchronous act. Following this, I am assuming that in order to avoid queueing microtask, we should define IS_REACT_ACT_ENVIRONMENT global.

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

Lines 55 to 77 in d27c1ac

	const root = ReactNoop.createRoot();
	// First test what happens without wrapping in act. This update would
	// normally be queued in a microtask.
	global.IS_REACT_ACT_ENVIRONMENT = false;
	ReactNoop.unstable_runWithPriority(DiscreteEventPriority, () => {
	root.render('A');
	});
	// Nothing has rendered yet
	expect(root).toMatchRenderedOutput(null);
	// Flush the microtasks by awaiting
	await waitForMicrotasks();
	expect(root).toMatchRenderedOutput('A');
	// Now do the same thing but wrap the update with `act`. No
	// `await` necessary.
	global.IS_REACT_ACT_ENVIRONMENT = true;
	act(() => {
	ReactNoop.unstable_runWithPriority(DiscreteEventPriority, () => {
	root.render('B');
	});
	});
	expect(root).toMatchRenderedOutput('B');

Validated with running tests on #28152 with these changes.
Also validated that it works for regression tests for previous versions of React.

[rickhanlonii]

Sick! Nice find!