test/gcp: Run cleanup function at the end

All EXIT traps are cleared on line 280 so the cleanup trap is never run
and VMs are waiting for 4 hours to get cleaned by
scheduled-cloud-cleaner. Run the cleanup at the end and rely on
scheduled-cloud-cleaner only in case of failures before that.

test/cases/gcp.sh

@@ -278,6 +278,8 @@ get_compose_metadata "$COMPOSE_ID"
 # Kill the journal monitor immediately and remove the trap
 sudo pkill -P ${WORKER_JOURNAL_PID}
 trap - EXIT
+# trap cleanup again
+trap cleanup EXIT
 
 # Did the compose finish with success?
 if [[ $COMPOSE_STATUS != FINISHED ]]; then