Command processing (2/n): testing

Summary:
*Stack summary*: this stack refactors plugin management actions to perform them in a dispatcher rather than in the root reducer (store.tsx) as all of these actions has side effects. To do that, we store requested plugin management actions (install/update/uninstall, star/unstar) in a queue which is then handled by pluginManager dispatcher. This dispatcher then dispatches all required state updates.

*Diff summary*: refactored Flipper mocking helpers to allow testing of plugin commands, and wrote some tests for pluginManager.

Reviewed By: mweststrate

Differential Revision: D26450344

fbshipit-source-id: 0e8414517cc1ad353781dffd7ffb4a5f9a815d38

desktop/app/src/reducers/__tests__/sandyplugins.node.tsx

@@ -250,7 +250,9 @@ test('it can send messages from sandy clients', async () => {
 });
 
 test('it should initialize "Navigation" plugin if not enabled', async () => {
-  const {client, store} = await createMockFlipperWithPlugin(TestPlugin);
+  const {client, store} = await createMockFlipperWithPlugin(TestPlugin, {
+    supportedPlugins: ['Navigation'],
+  });
 
   const Plugin2 = new _SandyPluginDefinition(
     TestUtils.createMockPluginDetails({