tests: image: fix incorrect assertion test

Python v3.12 and later raise an exception for this line with the
following message:
"""
AttributeError: 'called_once_with' is not a valid assertion. Use a spec for the mock if 'called_once_with' is meant to be an attribute.. Did you mean: 'assert_called_once_with'?
"""

A bit of research returned commit 1d4d677d1c90 ("gh-100690: Raise an
AttributeError when the assert_ prefix is forgotten when using Mock
(#100691)") from CPython git history, which in turns points at
python/cpython#100690.

This GitHub issue mentions that called_once_with returns a Mock object
instead of doing a proper assertion. This test is basically a no-op.

Therefore, let's fix the test by doing a proper assertion.

Fixes: 94d64c8 ("test: image: improve code coverage by testing BaseImage")
Signed-off-by: Quentin Schulz <[email protected]>

test/test_image.py

@@ -46,7 +46,7 @@ def test_copy_filepath_remove_superficial_options(
     ):
         base = BaseImage({"name": "test.jpg", "resize": "50%", "test": "test123"}, {})
         copy = base.copy()
-        mock_rm_sup_opt.called_once_with(
+        mock_rm_sup_opt.assert_called_once_with(
             {"name": "test.jpg", "resize": "50%", "test": "test123"}
         )
         assert copy == "test-%s-100x150.jpg" % (