Exit tests should suppress `SIGPIPE` on back channel pipes

[grynspan]

Exit tests write over pipes to send data to/from the child process and should suppress SIGPIPE to avoid crashing either process on an edge case.

[grynspan]

There's not really a viable way to ignore SIGPIPE only on the pipes we own, but we can look at switching to a Unix socket instead for the back channel and passing MSG_NOSIGNAL as needed.