tests: skip seccomp_filter_tsync if __NR_seccomp isn't defined

This change does two things to be more defensive: 1. add a checkskip script
which fails if __NR_seccomp isn't defined, and 2. allows
seccomp_filter_tsync to compile even if it isn't defined (and then fails
"gracefully" with a skip message if it does).

The checkskip bits are necessary because zdtm.sh gets confused if the test
exits 0 without actually invoking criu.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>