Test multiple raft interface layers

Should arrive at the correct raft thickness too then.

Contributes to issue CURA-8915.

tests/TestBuildVolume.py

@@ -163,6 +163,7 @@ class TestComputeDisallowedAreasStatic:
 
 class TestUpdateRaftThickness:
     setting_property_dict = {"raft_base_thickness": {"value": 1},
+                             "raft_interface_layers": {"value": 2},
                              "raft_interface_thickness": {"value": 1},
                              "raft_surface_layers": {"value": 1},
                              "raft_surface_thickness": {"value": 1},
@@ -193,7 +194,7 @@ class TestUpdateRaftThickness:
 
         assert build_volume.getRaftThickness() == 0
         build_volume._updateRaftThickness()
-        assert build_volume.getRaftThickness() == 3
+        assert build_volume.getRaftThickness() == 4  # 1 base layer of 1mm, 2 interface layers of 1mm each, 1 top layer of 1mm.
         assert build_volume.raftThicknessChanged.emit.call_count == 1
 
     def test_adhesionIsNotRaft(self, build_volume: BuildVolume):