fix: support old act from react-dom

Fetching `act` from `react-dom/test-utils` is deprecated, but keep support for it for now.

src/test-utils.ts

@@ -1,7 +1,12 @@
-import { act } from "react";
+import * as React from 'react'
+import * as DeprecatedReactTestUtils from 'react-dom/test-utils'
+
 declare global {
   var IS_REACT_ACT_ENVIRONMENT: boolean;
 }
+
+const act = typeof React.act === 'function' ? React.act : DeprecatedReactTestUtils.act
+
 type Item = {
   callback: IntersectionObserverCallback;
   elements: Set<Element>;