Fix erp5 slow queries logs

ERP5 software release has some support for mariadb slow queries:

• run pt-query-digest daily
• check that the number of slow queries and the slowest slow query do not exceed some thresholds defined as instance parameters

but this had issues:

• the daily pt-query-digest reports where kept as not-compressed text files, which over time takes quite a lot of disk space
• the threshold detection was not working, since strings and floats were compared - see https://lab.nexedi.com/nexedi/slapos.toolbox/merge_requests/83

This uses new techniques in software release testing from https://lab.nexedi.com/nexedi/slapos.core/merge_requests/245, we use faketime to simulate running crontabs and promises at a specific time in the future, so that we can assert the exact expected behaviour of crontab scripts.