selftests/namespaces: Skip efault tests when listns() is not available

selftests/namespaces: Skip efault tests when listns() is not available

When listns() is not implemented the iterator child detects ENOSYS and
exits cleanly with status PIDFD_SKIP before the parent has a chance to
signal it.  The parent sends SIGKILL (which is a harmless no-op at that
point) and then calls waitpid(), obtaining a normal-exit status.  The
subsequent ASSERT_TRUE(WIFSIGNALED(status)) therefore fails, causing the
three EFAULT-focused tests to report FAIL rather than SKIP on kernels that
do not yet carry listns() support.

After collecting the iterator's exit status, check whether it exited with
PIDFD_SKIP and issue a SKIP verdict in that case, consistent with the
behaviour of every other listns test that already handles ENOSYS correctly.

Signed-off-by: Ricardo B. Marlière <rbm@suse.com>
Link: https://patch.msgid.link/20260407-selftests-namespaces_fixes-v1-3-59109909d88b@suse.com
Signed-off-by: Christian Brauner <brauner@kernel.org>