support build aux coord with rollaxis

lib/iris/tests/unit/fileformats/nc_load_rules/helpers/test_build_auxiliary_coordinate.py

@@ -96,16 +96,24 @@ def _get_per_test_bounds_var(_coord_unused):
         )
 
     @classmethod
-    def _make_array_and_cf_data(cls, dimension_names):
+    def _make_array_and_cf_data(cls, dimension_names, rollaxis=False):
         shape = tuple(cls.dim_names_lens[name] for name in dimension_names)
         cf_data = mock.MagicMock(_FillValue=None, spec=[])
         cf_data.chunking = mock.MagicMock(return_value=shape)
-        data = np.arange(np.prod(shape), dtype=float).reshape(shape)
+        data = np.arange(np.prod(shape), dtype=float)
+        if rollaxis:
+            shape = shape[1:] + (shape[0],)
+            data = data.reshape(shape)
+            data = np.rollaxis(data, -1)
+        else:
+            data = data.reshape(shape)
         return data, cf_data
 
-    def _make_cf_bounds_var(self, dimension_names):
+    def _make_cf_bounds_var(self, dimension_names, rollaxis=False):
         # Create the bounds cf variable.
-        bounds, cf_data = self._make_array_and_cf_data(dimension_names)
+        bounds, cf_data = self._make_array_and_cf_data(
+            dimension_names, rollaxis=rollaxis
+        )
         bounds *= 1000  # Convert to metres.
         cf_bounds_var = mock.Mock(
             spec=CFVariable,
@@ -121,8 +129,10 @@ def _make_cf_bounds_var(self, dimension_names):
 
         return cf_bounds_var
 
-    def _check_case(self, dimension_names):
-        self.cf_bounds_var = self._make_cf_bounds_var(dimension_names=dimension_names)
+    def _check_case(self, dimension_names, rollaxis=False):
+        self.cf_bounds_var = self._make_cf_bounds_var(
+            dimension_names, rollaxis=rollaxis
+        )
 
         # Asserts must lie within context manager because of deferred loading.
         build_auxiliary_coordinate(self.engine, self.cf_coord_var)
@@ -140,7 +150,7 @@ def test_fastest_varying_vertex_dim__normalise_bounds(self):
 
     def test_slowest_varying_vertex_dim__normalise_bounds(self):
         # Bounds in the first (slowest varying) dimension.
-        self._check_case(dimension_names=("nv", "foo", "bar"))
+        self._check_case(dimension_names=("nv", "foo", "bar"), rollaxis=True)
 
     def test_fastest_with_different_dim_names__normalise_bounds(self):
         # Despite the dimension names ('x', and 'y') differing from the coord's