pgsql: Stabilize output of new isolation test insert-conflict-do-update

Stabilize output of new isolation test insert-conflict-do-update-4.

The test added by commit 4b760a181 assumed that a table's physical
row order would be predictable after an UPDATE. But a non-heap table
AM might produce some other order. Even with heap AM, the assumption
seems risky; compare a3fd53bab for instance. Adding an ORDER BY is
cheap insurance and doesn't break any goal of the test.

Author: Pavel Borisov <pashkin(dot)elfe(at)gmail(dot)com>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/CALT9ZEHcE6tpvumScYPO6pGk_ASjTjWojLkodHnk33dvRPHXVw@mail.gmail.com
Backpatch-through: 14

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/25b67fa7daa2d8d5452a1c9415ed16914ac6ba15

Modified Files
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src/test/isolation/expected/insert-conflict-do-update-4.out | 8 ++++----
src/test/isolation/specs/insert-conflict-do-update-4.spec | 2 +-
2 files changed, 5 insertions(+), 5 deletions(-)