test for post usage when combining usage items

to address this issue:
alan-turing-institute/rctab-functions#42

tests are empty for now

 On branch tests_for_post_usage
 Changes to be committed:
	modified:   tests/test_routes/test_usage.py

Author: dlpbc

tests/test_routes/test_usage.py

@@ -83,6 +83,12 @@ def test_post_usage(
         )
 
 
+def test_post_usage2(
+    mocker: pytest_mock.MockerFixture,
+) -> None:
+    del mocker
+
+
 def test_write_usage(
     app_with_signed_billing_token: Tuple[FastAPI, str], mocker: MockerFixture
 ) -> None: