Test accuracy of data reported by JFR events, where possible.

Considerations:
- there are many JFR events, need to prioritize them
- pick several events to start with
- for some (or many) events/cases it would be too difficult or impossible to test this
- need to think what to use as reference values or sources of truth
- for some values will need to use a margin of error

Possible sources of truth for events:
  - logs
  - data reported by JCMD
  - data obtained from the system