pgsql: Improve test case stability

Improve test case stability

Given unlucky timing, some of the new tests added by commit bc32a12e0db2
can fail spuriously.  We haven't seen such failures yet in buildfarm,
but allegedly we can prevent them with this tweak.

While at it, remove an unused injection point I (Álvaro) added.

Author: Mihail Nikalayeu <mihailnikalayeu@gmail.com>
Discussion: https://postgr.es/m/CADzfLwUc=jtSUEaQCtyt8zTeOJ-gHZ8=w_KJsVjDOYSLqaY9Lg@mail.gmail.com
Discussion: https://postgr.es/m/CADzfLwV5oQq-Vg_VmG_o4SdL6yHjDoNO4T4pMtgJLzYGmYf74g@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/417ac9c1eeebc5dd8186a9089055b8af43b2450e

Modified Files
--------------
src/backend/executor/execIndexing.c                 |  4 +---
src/backend/utils/time/snapmgr.c                    |  1 +
.../index-concurrently-upsert-predicate.out         | 11 ++++++++---
.../expected/index-concurrently-upsert.out          | 11 ++++++++---
.../specs/index-concurrently-upsert-predicate.spec  | 21 ++++++++++++++-------
.../specs/index-concurrently-upsert.spec            | 19 +++++++++++++------
6 files changed, 45 insertions(+), 22 deletions(-)