qtest timeouts on cross-i686-tci job

The qtest-ppc/boot-serial-test seems to sometimes time out on the cross-i686-tci job, eg:

 45/257 qemu:qtest+qtest-ppc / qtest-ppc/boot-serial-test                  TIMEOUT        180.20s   killed by signal 15 SIGTERM

https://gitlab.com/qemu-project/qemu/-/jobs/5890481086 is an example of it just scraping in under the wire at 180s.