tests: update exit code in case of push failing because it'd create new heads
CORE-TEST-OUTPUT-UPDATE: 9261f6c1d39b
This instance will be upgraded to Heptapod 18.0.1 on 2025-07-10 between 14:00 and 15:00 UTC+2
CORE-TEST-OUTPUT-UPDATE: 9261f6c1d39b