Speed up the fork_exec test

Previouly we run 500 times "python" process, each one running one
"sleep" process. This took more then 100 seconds on my laptop.

Now we run one "python" process, which starts 500 "sleep" processes.
This has the same effect, (doing 500 forks), but takes about 1.5 seconds
on the same machine.

Prviously the test stopped up to 1 seconds after a failure. Now the test
wait up to 10 seconds after a failure. This delay is is caused by
TestProcess and wait_for_strings not detecting and handling abnormal
termination of the process. However, since failures are unlikely, this
is extra delay is very important.
One of those days again ...
Use true ;)
Update README.rst