selftests/seccomp: Prepare for exclusive seccomp flags

Some seccomp flags will become exclusive, so the selftest needs to
be adjusted to mask those out and test them individually for the "all
flags" tests.

Cc: stable@vger.kernel.org # v5.0+
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Tycho Andersen <tycho@tycho.ws>
Acked-by: James Morris <jamorris@linux.microsoft.com>