procstat/tests: Fix flakiness by waiting for program to start

[jilles]

Summary

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

Test Plan

sh -c 'while kyua test; do :; done'
and let it continue for a while

[jilles]

Remove no longer used variable MAX_TRIES

[ngie]

Thank you for fixing my flaky test (and for teaching me a new tidbit about shell support)!