Add test for pluginStates

Summary:
It's not super useful, to be honest, but it's to have one
open source `.node.js` test in place so `yarn test` doesn't
error out.

Reviewed By: danielbuechele

Differential Revision: D10217530

fbshipit-source-id: c0e17e48782174b856ef6d5fe94c5c042564e4f5

src/reducers/__tests__/pluginStates.node.js

@@ -0,0 +1,16 @@
+/**
+ * Copyright 2018-present Facebook.
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ * @format
+ */
+
+import {default as reducer, setPluginState} from '../pluginStates';
+
+test('reduce setPluginState', () => {
+  const res = reducer(
+    {},
+    setPluginState({pluginKey: 'myPlugin', state: {a: 1}}),
+  );
+  expect(res).toEqual({myPlugin: {a: 1}});
+});