pathlib tests: create `walk()` test hierarchy without using class under test (#128338)

pathlib tests: create `walk()` test hierarchy without using class under test (#128338)

In the tests for `pathlib.Path.walk()`, avoid using the path class under
test (`self.cls`) in test setup. Instead we use `os` functions in
`test_pathlib`, and direct manipulation of `DummyPath` internal data in
`test_pathlib_abc`.