test: ipc: add receive_from_local

Very basic interface stuff. The goal is to have every nook and cranny of the
Ipc syscall covered so that those tests can be modified to instead call Lipc
in another test case, or variant suite (if that mechanism turns out to be sane
enough).

Next up: the sibling test, receive_from_foreign.
test: self: add test for fork_tid() with external IPC flows

fork_tid() shouldn't be using wildcards, but it does regardless. This test
teases that problem out.
testbench: remove wildcard receive from fork_tid()

It would break an external IPC flow, which it shouldn't have. This solution
starts a fresh thread that's known not to be a party to such flows, so it can
call Wait and be guaranteed only to receive from the actual child process.
test: self: remove #if 0'd diag()s

Minor cleanups.
test: ipc: add receive_from_foreign

Passes, as expected. During development a bug was found in fork_tid(), a
provocation test developed, and a solution written (though not in that order).
The microkernel itself was behaving correctly.