MFC r351819: procstat/tests: Fix flakiness by waiting for program to start
Some of the procstat tests start a program "while1" and examine the process
using procstat, but did not wait properly for it to start (kill -0 will
succeed immediately after the child process has been created).
Instead, have "while1" write something when it starts, and use a fifo to
wait for that.
PR: 233587, 233588