seccomp_bpf_tests: add support for proposed TSYNC behavior

Add support for the proposed cross-thread synchronization
functionality.  If implemented as proposed, it will provide
a means for a thread group to synchronize on a given seccomp
filter after thread creation as long as only the caller has
attached new filters (and any eligible threads are under seccomp
mode=2)

(Issuing as a pull request to allow easy commenting)