[examinador] Check query is ready to take a backup before starting tests

What is the problem?
In MB-61550 query was changed so that it returns an error from the backup endpoint if the migration from pre-7.6/_system scope metadata storage has not happened yet. This means that there might be a brief period when a >=7.6 first starts where backups fail with

Error backing up cluster: internal server error executing 'GET' request to '/api/v1/backup': {"_level":"exception","caller":"admin_accounting_endpoint:1011","cause":{"possible_reasons":["Migration active."]},"code":7300,"key":"service.backup_not_possible","message":"Backup not possible currently."}

Source

We seem to hit this pretty reliably in examinador which makes sense as we start a cluster, load very few items and then immediately take a backup.

What is the solution?
As part of the cluster readiness check we should wait until the query metadata migration completes.