bpo-45207: Make test_gdb.test_pycfunction() quiet (GH-28355) (GH-28365)

test_gdb.test_pycfunction() now ignores gdb stderr, it no longer logs
messages like:

    Function "meth_varargs" not defined.
(cherry picked from commit 84a6061e29e9dc13909bdf6f541f48c2a4f1d410)

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