selftests/seccomp: Add test for unknown SECCOMP_RET kill behavior

While we were testing for the behavior of unknown seccomp filter return
values, there was no test for how it acted in a thread group. Add a test
in the thread group tests for this.

Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Kees Cook <keescook@chromium.org>