pgsql: Fix buildfarm failure introduced by 6168c65ddc.

Fix buildfarm failure introduced by 6168c65ddc.

Commit 6168c65ddc added a test verifying that the conflict detection
slot's xmin regresses to the horizon pinned by an in-progress transaction
in the database of a newly added subscription that has retain_dead_tuples
enabled. However, it used query_until(), which can return before starting
a transaction, allowing the test to proceed without a pinned xid.

Use query_safe() so the transaction setup completes synchronously and the
xid is assigned before the xmin checks run.

Per buildfarm.

Author: Hayato Kuroda <kuroda(dot)hayato(at)fujitsu(dot)com>
Discussion: https://postgr.es/m/OS9PR01MB12149257A97EE1BD3296F6AEBF5B12@OS9PR01MB12149.jpnprd01.prod.outlook.com
Backpatch-through: 19

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b3dfd8d4ea1547ccbb6fca64f0334e40762b0f4d

Modified Files
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src/test/subscription/t/035_conflicts.pl | 10 ++--------
1 file changed, 2 insertions(+), 8 deletions(-)