Tests for plugin installer with in-memory file system (#1761)

Summary:
Pull Request resolved: https://github.com/facebook/flipper/pull/1761

Plugin installed logic is quite complex and heavily relies on file system. So I added integration tests which uses in-memory file system. Here I've covered complex functions added in the previous diff of this stack.

Reviewed By: mweststrate

Differential Revision: D25393473

fbshipit-source-id: a823eb7d6707152a1c3717a6da25b6beee85801d

desktop/app/package.json

@@ -82,7 +82,7 @@
     "@types/lodash.memoize": "^4.1.6",
     "flipper-test-utils": "0.0.0",
     "metro-runtime": "^0.63.0",
-    "mock-fs": "^4.12.0",
+    "mock-fs": "^4.13.0",
     "pretty-format": "^26.1.0",
     "react-refresh": "^0.8.1",
     "redux-mock-store": "^1.0.1",