Added unit test for parasitic error

tests/test_stage_measure.py

@@ -1,8 +1,11 @@
 """File contains unit tests for stage_measure."""
 
+import pytest
 from openflexure_microscope_server.things.stage_measure import (
     _generate_move_dicts,
     _predict_z,
+    _parasitic_detect,
+    ParasiticMotionError
 )
 
 from .mock_things.mock_csm import MockCSMThing
@@ -45,3 +48,10 @@ def test_predict_z():
     )
     expected_z_diff = 1343.1625053206606
     assert mock_z_diff == expected_z_diff
+
+def test_parasitic_detect():
+    """Check that the parasitic error is raised correctly."""
+    mock_delta = 100
+    mock_max = 50
+    with pytest.raises(ParasiticMotionError):
+        _parasitic_detect(delta=mock_delta, max_allowed_delta=mock_max)