test: string: combine simple/multi compound send/receive tests
test: string: add build_stritem(), use it in echo_compound_{send,recv}

Unit test refactoring, in my C program?
test: string: add multiple stringitem test

Also move the fancy stringitem stuff down after the xferfault & holes tests.
mapdb: return proper given-rights mask in mapdb_map_pages() complex case
testbench: move x{start,join}_thread() into util.c
ipc: handle RcvWindow/SndBase correctly

Removes a panic case.
testbench: ipc: add test of mapping into small or large acceptor

These tests could be bottled up better, and they could also have some kind of
a check that the right reception address is computed by the receiver in the
large acceptor case. (an extra untyped word in the reply, perhaps.)