Complete first pass (untested on hardware) of bg detect refactor.

tests/mock_things/mock_camera.py

@@ -0,0 +1,28 @@
+"""Testing submodule with mock Camera Things.
+
+These mocks are designed to be inserted as dependencies to provide specific
+functionality and return values.
+
+The mocks do not subclass Things. Instead, they return predefined
+answers to functions.
+"""
+
+from openflexure_microscope_server.background_detect import (
+    BackgroundDetectorStatus,
+    ColourChannelDetectSettings,
+)
+
+
+class MockCameraThing:
+    """A mock background detect Thing that imports no code from BackgroundDetectThing.
+
+    The class needs functionality added to it over time as more complex
+    mocking is needed. It imports no code from BackgroundDetectThing so that coverage
+    is not artificially inflated.
+    """
+
+    background_detector_status = BackgroundDetectorStatus(
+        ready=True,
+        settings=ColourChannelDetectSettings(),
+        settings_schema={"fake": "schema"},
+    )