selftests/seccomp: Allow get_metadata to XFAIL

Since seccomp_get_metadata() depends on CHECKPOINT_RESTORE, XFAIL the
test if the ptrace reports it as missing.

Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Tycho Andersen <tycho@tycho.ws>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>