[3.12] Make test_fcntl quiet (GH-108758) (#108760)

Make test_fcntl quiet (GH-108758)

Running test_fcntl logs two "struct.pack: ..." lines because
multiprocessing imports test_fcntl twice with
test.support.verbose=1. Move get_lockdata() inside TestFcntl test
case and only call it where it's needed, to stop logging these lines.
(cherry picked from commit 23f54c120067c96973619b9501fe4dff1b055188)

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