Add stress test for spawning processes

[talex5]

This spawns 10,000 subprocesses (100 at a time) running sh -c 'echo N' and checks they each give the correct output.

Interestingly, the eio_posix backend is slightly faster than eio_linux:

$ EIO_BACKEND=posix dune exec -- ./stress/stress_proc.exe
+Finished process stress test: ran 10000 processes in 2.75s

$ EIO_BACKEND=linux dune exec -- ./stress/stress_proc.exe
+Finished process stress test: ran 10000 processes in 3.31s

Someone might like to run that under magic-trace and find out why, as part of #450.