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/RCTTest/React-RCTTest.podspec

@@ -0,0 +1,35 @@
+# coding: utf-8
+# 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.
+
+require "json"
+
+package = JSON.parse(File.read(File.join(__dir__, "..", "..", "package.json")))
+version = package['version']
+
+source = { :git => 'https://github.com/facebook/react-native.git' }
+if version == '1000.0.0'
+  # This is an unpublished version, use the latest commit hash of the react-native repo, which we’re presumably in.
+  source[:commit] = `git rev-parse HEAD`.strip
+else
+  source[:tag] = "v#{version}"
+end
+
+Pod::Spec.new do |s|
+  s.name                   = "React-RCTTest"
+  s.version                = version
+  s.summary                = "Tools for integration and snapshot testing."
+  s.homepage               = "http://facebook.github.io/react-native/"
+  s.license                = package["license"]
+  s.author                 = "Facebook, Inc. and its affiliates"
+  s.platforms              = { :ios => "9.0", :tvos => "9.2" }
+  s.source                 = source
+  s.source_files           = "**/*.{h,m}"
+  s.preserve_paths         = "package.json", "LICENSE", "LICENSE-docs"
+  s.framework              = "XCTest"
+  s.header_dir             = "React"
+
+  s.dependency "React-Core", version
+end