Amend tests to catch datetime not formatted to ISO8601.

tests/tests_unit/test_unit.py

@@ -34,13 +34,15 @@ def test_datetime_string_replacement(self, key, underscore_position):
             regex = re.compile(rf"{start}_time-\d\d\d\d\d\d_{end}")
         elif key == tags("datetime"):
             regex = re.compile(
-                rf"{start}_date-\d\d\d\d\d\d\d\d_time-\d\d\d\d\d\d_{end}"
+                rf"{start}_datetime-\d\d\d\d\d\d\d\dT\d\d\d\d\d\d_{end}"  # TODO: can we use something like "[0-9]+" with a specific number?
             )
 
         name_list = [name]
         formatting.update_names_with_datetime(name_list)
 
-        assert re.search(regex, name_list[0]) is not None
+        assert (
+            re.search(regex, name_list[0]) is not None
+        ), "datetime formatting is incorrect."
 
     @pytest.mark.parametrize(
         "prefix_and_names",