Refactor generate-specs-cli and add tests

Summary:
This Diff splits the `generate-specs-cli.js` script into:
* `generate-specs-cli-executor.js`: which contains the logic to generate the code for iOS.
* `generate-specs-cli.js`: which contains the argument parsing logic and invokes the executor.

Finally it introduces some tests.

## Changelog
[iOS][Changed] - Refactor part of the codegen scripts and add tests.

Reviewed By: cortinico, dmitryrykun

Differential Revision: D35892576

fbshipit-source-id: 6a3205af049bf55aa4ecaf64544ebc4eb7b13f73

scripts/codegen/codegen-utils.js

@@ -0,0 +1,37 @@
+/**
+ * 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.
+ *
+ * @format
+ */
+
+'use strict';
+
+/**
+ * Wrapper required to abstract away from the actual codegen.
+ * This is needed because, when running tests in Sandcastle, not everything is setup as usually.
+ * For example, the `react-native-codegen` lib is not present.
+ *
+ * Thanks to this wrapper, we are able to mock the getter for the codegen in a way that allow us to return
+ * a custom object which mimics the Codegen interface.
+ *
+ * @return an object that can generate the code for the New Architecture.
+ */
+function getCodegen() {
+  let RNCodegen;
+  try {
+    RNCodegen = require('../../packages/react-native-codegen/lib/generators/RNCodegen.js');
+  } catch (e) {
+    RNCodegen = require('../react-native-codegen/lib/generators/RNCodegen.js');
+  }
+  if (!RNCodegen) {
+    throw 'RNCodegen not found.';
+  }
+  return RNCodegen;
+}
+
+module.exports = {
+  getCodegen: getCodegen,
+};