added tests to createPlugin, plus minor refactors

Co-authored-by: Patrik Oldsberg <poldsberg@gmail.com>
Co-authored-by: Johan Haals <johan.haals@gmail.com>
Signed-off-by: Fredrik Adelöw <freben@gmail.com>

packages/frontend-plugin-api/package.json

@@ -24,7 +24,9 @@
   },
   "devDependencies": {
     "@backstage/cli": "workspace:^",
-    "@testing-library/jest-dom": "^5.10.1"
+    "@backstage/frontend-app-api": "workspace:^",
+    "@testing-library/jest-dom": "^5.10.1",
+    "@testing-library/react": "^12.1.3"
   },
   "files": [
     "dist"