Make sure Sandy Device Plugins can be unit testsed

Summary: Add unit tests to verify that the unit test utilities for for Sandy device plugins work as expected. Fixed a bug revealed by that and cleaned up some TODO's

Reviewed By: jknoxville, passy, nikoant

Differential Revision: D22693928

fbshipit-source-id: 93162db19d826d0cd7f642cef1447fd756261ac8

desktop/flipper-plugin/src/test-utils/test-utils.tsx

@@ -251,7 +251,7 @@ export function startDevicePlugin<Module extends FlipperDevicePluginModule>(
     createMockPluginDetails(),
     module,
   );
-  if (definition.isDevicePlugin) {
+  if (!definition.isDevicePlugin) {
     throw new Error(
       'Use `startPlugin` or `renderPlugin` to test non-device plugins',
     );