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/pkg-lib/package.json

@@ -21,7 +21,7 @@
     "flipper-test-utils": "0.0.0",
     "globby": "^11",
     "jest": "^26",
-    "mock-fs": "^4.12.0",
+    "mock-fs": "^4.13.0",
     "prettier": "^2.0.0",
     "rimraf": "^3.0.2",
     "ts-jest": "^26.0.0",