[3.14] gh-133454: Reduce the number of threads in test_racing_getbuf_and_releasebuf (GH-133458) (GH-134589)

The original reproducer only used 10 threads.
(cherry picked from commit fc0c9c24121c1a62b25b79062286f976699b59e9)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>