bpo-44860: Update test_sysconfig for posix_user platlib (GH-28235) (GH-28251)

Update test_sysconfig.test_user_similar() for the posix_user scheme:
"platlib" doesn't use sys.platlibdir.
(cherry picked from commit 49acac00c08838d8080ce00d02c05284b94f8fb2)

Co-authored-by: Victor Stinner <vstinner@python.org>