Move RCTTest & takeSnapshot to RNTester (#23721)

Summary:
Part of: #23313.

This moves the `RCTTest` lib from `Libraries/RCTTest` to `RNTester/RCTTest`. This also removes `takeSnapshot` from React Native, and implements it as a standalone module in RNTester called `ScreenshotManager`.

[General] [Removed] - RCTTest & ReactNative.takeSnapshot
Pull Request resolved: https://github.com/facebook/react-native/pull/23721

Differential Revision: D14434796

Pulled By: PeteTheHeat

fbshipit-source-id: d6e103a0ea0b6702701cdb5ce8449163ca4628ce

RNTester/NativeModuleExample/NativeScreenshotManager.js

@@ -0,0 +1,28 @@
+/**
+ * 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.
+ *
+ * @flow
+ * @format
+ */
+
+'use strict';
+
+import type {TurboModule} from 'RCTExport';
+import * as TurboModuleRegistry from 'TurboModuleRegistry';
+
+export interface Spec extends TurboModule {
+  +getConstants: () => {||};
+  takeSnapshot(id: string): Promise<string>;
+}
+
+const NativeModule = TurboModuleRegistry.get<Spec>('ScreenshotManager');
+
+export function takeSnapshot(id: string): Promise<string> {
+  if (NativeModule != null) {
+    return NativeModule.takeSnapshot(id);
+  }
+  return Promise.reject();
+}