gh-101334: Don't force USTAR format in test_tarfile. (GH-101572)

That causes the test to fail when run using a high UID as that ancient format
cannot represent it. The current default (PAX) and the old default (GNU) both
support high UIDs.
(cherry picked from commit ffcb8220d7a8c8ca169b467d9e4a752874f68af2)

Co-authored-by: Gregory P. Smith <greg@krypto.org>