Change llprocesslauncher_test.cpp eyeballing to program verification.

That is, where before we just flung stuff to stdout with the expectation that
a human user would verify, replace with assertions in the test code itself.
Quiet previous noise on stdout.
Introduce a temp script file that produces output on both stdout and stderr,
with sleep() calls so we predictably have to wait for it. Track and then
verify the history of our interaction with the child process, noting
especially EWOULDBLOCK attempts.