tests: get rid of HAVE_SECCOMP nonsense

Because of the #undef (probably left over from some debugging I did that
accidentally got committed), this test never ran. Anyway, this is all
unnecessary because we can just use the seccomp_filter feature flag in the
tests.

Also reenable the seccomp_filter_tsync test.

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