fix: ensuring that dashboards dir exists in all tests

[NikolaMilosa]

This PR ensures that both k8s and farm based testnets have a directory for grafana dashboards (even if they are empty). This is to prevent having log lines like the following:

2024-12-20 03:22:31.400 INFO[uvms_logs_stream:StdOut] [uvm=prometheus] JournalRecord {message: "logger=provisioning.dashboard type=file name=provisioned-grafana-dashboards t=2024-12-20T03:22:30.89689519Z level=error msg="Cannot read directory" error="stat /config/grafana/dashboards: no such file or directory"", system_unit: "grafana.service", comm: "grafana"}