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
Reviewed by: ngie
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D21519