fix up 2 failining unit tests

Author: walshmm

src/snapred/backend/data/GroceryService.py

@@ -1520,8 +1520,8 @@ def combinePixelMasks(self, outputMaskWsName: WorkspaceName, masks2Combine: List
             raise ValueError("Internal Error: Lists of masks to combine is empty")
 
         if not self.mantidSnapper.mtd.doesExist(outputMaskWsName):
-            raise ValueError(("Internal Error: outputMaskWs should exist before attempting to combine masks with it."
-                              "consider using fetchCompatiblePixelMask to generate it."))
+            raise ValueError(("Internal Error: outputMaskWs should exist before attempting to combine masks with it. "
+                              "Consider using fetchCompatiblePixelMask to generate it."))
 
         for maskWsName in masks2Combine:
             if maskWsName == outputMaskWsName:

tests/unit/backend/service/test_CalibrationService.py

@@ -766,6 +766,8 @@ def test_fetchDiffractionCalibrationGroceries(self):
             }
         )
         self.instance.groceryService.fetchCompatiblePixelMask = mock.Mock(side_effect=lambda n, _, __: n)
+        self.instance.groceryService.mantidSnapper.mtd.doesExist = mock.Mock(side_effect=lambda _: True)
+        self.instance.groceryService.mantidSnapper.BinaryOperateMasks = mock.Mock()
         self.instance.validateRequest = mock.Mock()
 
         request = mock.Mock(
@@ -805,7 +807,8 @@ def test_fetchDiffractionCalibrationGroceries(self):
                 combinedMask,
                 [calibrationMaskName, "mask1", "mask2", combinedMask],
             )
-
+            assert self.instance.groceryService.mantidSnapper.BinaryOperateMasks.called
+            
     @mock.patch(thisService + "SimpleDiffCalRequest", spec_set=SimpleDiffCalRequest)
     def test_diffractionCalibration_calls_others(self, SimpleDiffCalRequest):
         # mock out external functionalties
@@ -1101,6 +1104,8 @@ def test_diffractionCalibration_with_bad_masking(
         self.instance.groceryService.fetchGroceryDict = mock.Mock(return_value={"maskWorkspace": self.sampleMaskWS})
         self.instance.groceryService.getWorkspaceForName = mock.Mock(side_effect=[False, mtd[self.sampleMaskWS]])
         self.instance.groceryService.fetchCompatiblePixelMask = mock.Mock(side_effect=lambda n, _, __: n)
+        self.instance.groceryService.mantidSnapper.mtd.doesExist = mock.Mock(side_effect=lambda _: True)
+        self.instance.groceryService.mantidSnapper.BinaryOperateMasks = mock.Mock()
         # Call the method with the provided parameters
         request = DiffractionCalibrationRequest(
             runNumber="123",