Fix isolation test to be less timing-dependent

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

Commit: 0f38a1d85abebd79ad8ac348e3912463ae56192e
Author: Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: 2018-01-03T15:06:56Z
Releases: 9.6.7
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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