Fix isolation test to be less timing-dependent

Alvaro Herrera <alvherre@alvh.no-ip.org>

Commit: 87c44b1aa36e78e67f7529fe523e3aa0fb821730
Author: Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2018-01-03T15:07:58Z
Releases: 10.2
Fix isolation test to be less timing-dependent

I did this by adding another locking process, which makes the other two
wait.  This way the output should be stable enough.

Per buildfarm and Andres Freund
Discussion: https://postgr.es/m/20180103034445.t3utrtrnrevfsghm@alap3.anarazel.de

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