selftests/nolibc: prevent coredumps during test execution

The child process forked during stackprotector tests intentionally gets
killed with SIGABRT. By default this will trigger writing a coredump.
The writing of the coredump can spam the systems coredump machinery and
take some time.

Timings for the full run of nolibc-test:
Before: 200ms
After:   20ms

This is on a desktop x86 system with systemd-coredumpd enabled.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>