gh-122864: Fix a ``test_funcattrs.test___builtins__`` when executing directly (#124845)

Previously when executing ``test_functattrs.test___builtins__`` directly, it failed because the fact, that ``__builtins__`` is refers to the built-in module ``builtins`` while it's expects a ``__builtins__.__dict__``. But when this test is being run from another module, then ``__builtins__`` is refers to ``builtins.__dict__``. Now this part of the behaviour is covered.
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Co-authored-by: Victor Stinner <vstinner@python.org>