Improve k6 usage

[carpawell]

Some improvements that can be applied:

1. Use checks, they are added in the summary and this is how basically we should use k6: https://k6.io/docs/using-k6/checks/
2. Use thresholds, they can fail a test earlier and in an automated manner (at least we may start to have some goals for the tested product): https://k6.io/docs/using-k6/thresholds/
3. Use non-constant executors, they allow simulating more controllable load (constant-arrival-rate is really nice IMO): https://k6.io/docs/using-k6/scenarios/#scenario-executors
4. Visualise summaries: https://github.com/benc-uk/k6-reporter
5. Visualise metrics, IMO that is the key reason we are using a loader, we should see if a service can handle the load flat or it almost dies at the end of a test: https://k6.io/docs/integrations/results-visualization/