add a test for the skeet-firestore plugin method

[euledge]

Details of Improvement

• I want to add a test for the method because there is no test written for this plugin.

Expected behavior

• Using Firebase Emulator Suit in a local environment.
• Testing is possible even in a CI environment.

Reference link

• https://firebase.google.com/docs/rules/unit-tests
• https://github.com/firebase/quickstart-testing/tree/master/unit-test-security-rules-v9