Fix mocking of md.preprocessors in tests

The `md.preprocessors` attribute wasn't being mocked correctly and
returned a new mock object instead of the custom mock object we wanted.

The tests passed anyway, because the only thing that was used was the
`highlight` return value, which evaluates to `True` when it is a new
mock object instead of a proper `bool` value, but now that the type of
the preprocessor is checked to be able to call `highlight`, the tests
failed.

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

tests/test_do_format.py

@@ -8,6 +8,7 @@
 from unittest import mock
 
 import pytest
+from pymdownx.superfences import SuperFencesBlockPreprocessor
 
 from frequenz.pymdownx.superfences.filter_lines import (
     LinesRange,
@@ -21,9 +22,10 @@
 def md_mock() -> Iterator[mock.MagicMock]:
     """Mock the `md` object."""
     md = mock.MagicMock()
-    preprocessor = mock.MagicMock()
-    preprocessor.highlight.side_effect = True
-    md.preprocessors.return_value = {"fenced_code_block": preprocessor}
+    preprocessor = mock.MagicMock(spec=SuperFencesBlockPreprocessor)
+    preprocessor.highlight.return_value = True
+    preprocessors = {"fenced_code_block": preprocessor}
+    md.preprocessors.__getitem__.side_effect = preprocessors.__getitem__
     yield md