s390x-ubuntu-2004-debug build failure rate over 90%

s390x-ubuntu-2004-debug builder is constantly failing: https://buildbot.mariadb.org/#/builders/677.

Most failures are due to timeouts.

If the builder is known to be slow, please increase test case timeout: mtr --testcase-timeout=1800.

If builder has hardware malfunction, it should be fixed?

It can also happen that the builder is alright and there's s390 specific bug in MariaDB, which makes it slow. If so, this issue should be transitioned to MDEV.