selftests/nolibc: Add coverage of vfork()

Generalise the existing fork() test to also cover the newly added vfork()
implementation.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20250703-arm64-gcs-vfork-exit-v3-4-1e9a9d2ddbbe@kernel.org


Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>