AthenaPoolUtilities: Add missing warning pattern in post_tpcnvtest.sh

Fixes spurious unit test failures in CI, like seen in !44516 (comment 4586583) and !44583 (comment 4591115).

Discussed with and suggested by @jchapman in "Merge Request shifter" mattermost channel:
https://mattermost.web.cern.ch/atlassoftware/pl/if8i74jfdbdntkfzjr8qy6qd3w

MR submitted from GitLab Web Editor, so the branch is on the main repo