Use TestDevice from flipper-frontend-core in flipper-ui-core

Summary: See D37139129

Reviewed By: lblasa

Differential Revision: D37235567

fbshipit-source-id: a0596ba71fcb8722088e51361774389a7daaebb8

desktop/flipper-frontend-core/src/index.tsx

@@ -16,6 +16,7 @@ export {
 } from './AbstractClient';
 export {default as ArchivedDevice} from './devices/ArchivedDevice';
 export {default as BaseDevice, DeviceExport} from './devices/BaseDevice';
+export {TestDevice} from './devices/TestDevice';
 export * from './globalObject';
 export * from './plugins';
 export {getPluginKey} from './utils/pluginKey';