Add internal-test-utils package

Our internal test utils are currently located in the jest-react
package. At one point, the idea was to publish some of these as
public, officially supported APIs, but we abandoned that idea in favor
of `act`. We should really just delete the jest-react package from
our repo.

What we can do instead is put our internal test utils into a private
package. This adds such a package, called internal-test-utils. There are
no exported APIs yet but I'll add some in the next step.

.eslintrc.js

@@ -325,6 +325,7 @@ module.exports = {
         'packages/react-native-renderer/**/*.js',
         'packages/eslint-plugin-react-hooks/**/*.js',
         'packages/jest-react/**/*.js',
+        'packages/internal-test-utils/**/*.js',
         'packages/**/__tests__/*.js',
         'packages/**/npm/*.js',
       ],

packages/internal-test-utils/ReactInternalTestUtils.js

@@ -0,0 +1,8 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+// TODO: Move `internalAct` and other test helpers to this package

packages/internal-test-utils/index.js

@@ -0,0 +1 @@
+export * from './ReactInternalTestUtils';

packages/internal-test-utils/package.json

@@ -0,0 +1,5 @@
+{
+  "private": true,
+  "name": "internal-test-utils",
+  "version": "0.0.0"
+}

scripts/jest/config.build.js

@@ -12,6 +12,12 @@ const NODE_MODULES_DIR =
 // Find all folders in packages/* with package.json
 const packagesRoot = join(__dirname, '..', '..', 'packages');
 const packages = readdirSync(packagesRoot).filter(dir => {
+  if (dir === 'internal-test-utils') {
+    // This is an internal package used only for testing. It's OK to read
+    // from source.
+    // TODO: Maybe let's have some convention for this?
+    return false;
+  }
   if (dir.charAt(0) === '.') {
     return false;
   }