make main.group_by MDEV-6129 UNION query deterministic

main.group_by is flaky in the mtr suite. The failing case is the regression test for MDEV-6129 (server crash on UNION with ORDER BY <expr> IS NULL):

SET sql_mode='ONLY_FULL_GROUP_BY';
SELECT 1 AS test UNION SELECT 2 AS test ORDER BY test IS NULL ASC;
Both rows have test IS NULL = 0, so the ORDER BY key is constant and the row order is unspecified. Under parallel test load (mtr --parallel=auto) the server sometimes returns 2,1 instead of the 1,2 baked into group_by.result:

--- mysql-test/main/group_by.result

+++ mysql-test/main/group_by.reject

@@ -2636,8 +2636,8 @@

 SET sql_mode='ONLY_FULL_GROUP_BY';

 SELECT 1 AS test UNION SELECT 2 AS test ORDER BY test IS NULL ASC;

 test

-1

 2

+1

 SET sql_mode='';

Repro seen recently on the mariadb-connector-c CI (parallel build, clean 11.4 tree): https://github.com/mariadb-corporation/mariadb-connector-c/actions/runs/24903791217/job/72927889893 — and on the scheduled 3.4 run of 2026-04-19 against the same 11.4 server (https://github.com/mariadb-corporation/mariadb-connector-c/actions/runs/24619796993/job/71988293365), which had no libmariadb changes at all.