feat: wrap waitFor in (async) act to support async queries without explicit act call (#344)

* Wrapped waitFor call in act to support async waitFor/findBy

* Updated docs

* Spelling mistake

* Added short info in migration guide

* Corrected spelling

* Clarified comments

* Added async act implementation conditional on React version

* Code review changes

* Update src/waitFor.js

Co-authored-by: Michał Pierzchała <thymikee@gmail.com>

* Update src/waitFor.js

Co-authored-by: Michał Pierzchała <thymikee@gmail.com>

* Fixed prettier

* Update src/waitFor.js

Co-authored-by: Michał Pierzchała <thymikee@gmail.com>

Co-authored-by: Michał Pierzchała <thymikee@gmail.com>

src/waitFor.js

@@ -1,16 +1,25 @@
 // @flow
 
+import * as React from 'react';
+import act from './act';
 import { throwRemovedFunctionError } from './helpers/errors';
 
 const DEFAULT_TIMEOUT = 4500;
 const DEFAULT_INTERVAL = 50;
 
+function checkReactVersionAtLeast(major: number, minor: number): boolean {
+  if (React.version === undefined) return false;
+  const [actualMajor, actualMinor] = React.version.split('.').map(Number);
+
+  return actualMajor > major || (actualMajor === major && actualMinor >= minor);
+}
+
 export type WaitForOptions = {
   timeout?: number,
   interval?: number,
 };
 
-export default function waitFor<T>(
+function waitForInternal<T>(
   expectation: () => T,
   options?: WaitForOptions
 ): Promise<T> {
@@ -38,6 +47,25 @@ export default function waitFor<T>(
   });
 }
 
+export default async function waitFor<T>(
+  expectation: () => T,
+  options?: WaitForOptions
+): Promise<T> {
+  if (!checkReactVersionAtLeast(16, 9)) {
+    return waitForInternal(expectation, options);
+  }
+
+  let result: T;
+
+  //$FlowFixMe: `act` has incorrect flow typing
+  await act(async () => {
+    result = await waitForInternal(expectation, options);
+  });
+
+  //$FlowFixMe: either we have result or `waitFor` threw error
+  return result;
+}
+
 export function waitForElement<T>(
   expectation: () => T,
   _timeout: number = 4500,