test: ipc: refine and split tid_spec_{from,to}
testbench: fix local/global TID confusion wrt thread_on_fork()

Intra-space IPC always returns a local sender ID.
ipc: record only global TIDs in <struct ipc_wait>

This complicates abort_thread_ipc(), which now has to resolve t->ipc_to; and
makes find_local_sender() much simpler.