TST: include Readback in test_client.py::test_apply

The test didn't consider Readback handling because sample_database doesn't
include any Readbacks. Rather than modify sample_database and possibly
affect other tests, I've added a minimal fixture for considering a single
Setpoint-Readback pair.

superscore/tests/conftest.py

@@ -680,6 +680,27 @@ def parameter_with_readback() -> Parameter:
     return setpoint
 
 
+@pytest.fixture(scope='function')
+def setpoint_with_readback() -> Setpoint:
+    """
+    A simple setpoint-readback value pair
+    """
+    readback = Readback(
+        uuid="7b30ddba-9fae-4691-988c-07384c29fe22",
+        pv_name="RBV",
+        description="A readback PV",
+        data=False,
+    )
+    setpoint = Setpoint(
+        uuid="418ed1ab-f1cf-4188-8f4c-ae7cbaf00e6c",
+        pv_name="SET",
+        description="A setpoint PV",
+        data=True,
+        readback=readback,
+    )
+    return setpoint
+
+
 @pytest.fixture(scope='function')
 def filestore_backend(tmp_path: Path) -> FilestoreBackend:
     fp = Path(__file__).parent / 'db' / 'filestore.json'

superscore/tests/test_client.py

@@ -56,7 +56,7 @@ def test_gather_data(mock_client, sample_database):
 
 
 @patch('superscore.control_layers.core.ControlLayer.put')
-def test_apply(put_mock, mock_client: Client, sample_database: Root):
+def test_apply(put_mock, mock_client: Client, sample_database: Root, setpoint_with_readback):
     put_mock.return_value = MockTaskStatus()
     snap = sample_database.entries[3]
     mock_client.apply(snap)
@@ -69,6 +69,10 @@ def test_apply(put_mock, mock_client: Client, sample_database: Root):
     mock_client.apply(snap, sequential=True)
     assert put_mock.call_count == 3
 
+    put_mock.reset_mock()
+    mock_client.apply(setpoint_with_readback, sequential=True)
+    assert put_mock.call_count == 1
+
 
 @patch('superscore.control_layers.core.ControlLayer._get_one')
 def test_snap(