GH-89812: Make symlink support configurable in pathlib tests. (GH-106060)

Adjust the pathlib tests to add a new `PathTest.can_symlink` class
attribute, which allows us to enable or disable symlink support in tests.
A (near-)future commit will add an `AbstractPath` class; its tests will
hard-code the value to `True` or `False` depending on a stub subclass's
capabilities.