pgsql: Fix isolation test to be less timing-dependent

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

Branch
------
REL9_6_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/0f38a1d85abebd79ad8ac348e3912463ae56192e

Modified Files
--------------
src/test/isolation/expected/multiple-cic.out | 17 +++++++++++------
src/test/isolation/specs/multiple-cic.spec   | 12 ++++++++----
2 files changed, 19 insertions(+), 10 deletions(-)