ipc: test fault occurrence in echo_with_hole

Also replace calloc with valloc and memset.
testbench: add rand32(), random_string()

Inspired by the similarly-named functions in the CCAN bdelta package's unit
test suite, though somewhat different in implementation.
ipc: add long simple string transfer test

Passes, but the "receiver in another space" case seems to trigger some sort of
an extra butt-slow mode in forkserv and is therefore commented out.