gh-94315: Check for DAC override capability (GH-94316)

``os.geteuid() == 0`` is not a reliable check whether the current user
has the capability to bypass permission checks. Tests now probe for DAC
override.
(cherry picked from commit 7e0d98ecb3f049841de9854e7a3eca3e2638e4b2)

Co-authored-by: Christian Heimes <christian@python.org>