Add unit tests for selection changes and plugin list computation

Summary: Tests to verify the more complex selection logic resulting from the Metro device exception. Also verifies the logic that computes the available plugins

Reviewed By: nikoant

Differential Revision: D24445555

fbshipit-source-id: 70110c4470e6aa1356e814aa40744b65c21cad89

desktop/app/src/test-utils/createMockFlipperWithPlugin.tsx

@@ -39,7 +39,7 @@ import {getPluginKey, isDevicePluginDefinition} from '../utils/pluginUtils';
 import {getInstance} from '../fb-stubs/Logger';
 import {setFlipperLibImplementation} from '../utils/flipperLibImplementation';
 
-type MockFlipperResult = {
+export type MockFlipperResult = {
   client: Client;
   device: BaseDevice;
   store: Store;