seccomp: add unit test for seccomp filtering

seccomp: add unit test for seccomp filtering

The handling of some syscalls / libc function is quite subtle. For
example, 'fork' at a libc level doesn't always correspond to 'fork'
at a syscall level, since the 'clone' syscall is preferred usually.

The unit test will help to detect these kind of problems. A point of
difficulty in writing a test though is that the QEMU build process may
already be confined by seccomp. For example, if running inside a
container. Since we can't predict what filtering might have been applied
already, we are quite conservative and skip all tests if we see any kind
of seccomp filter active.

Acked-by: Eduardo Otubo <otubo@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>