🎉Source Harvest: increase unit test coverage at least 90% (#15220)

* Increased unit test coverage

* Increased unit test

* Updated to linter

airbyte-integrations/connectors/source-harvest/unit_tests/conftest.py

@@ -10,7 +10,6 @@ from pytest import fixture
 def config_fixture(requests_mock):
     url = "https://id.getharvest.com/api/v2/oauth2/token"
     requests_mock.get(url, json={})
-
     config = {"account_id": "ID", "replication_start_date": "2021-01-01T21:20:07Z", "credentials": {"api_token": "TOKEN"}}
 
     return config