[MAINTENACE] Clarify usage and behavior of testing multiple CNFs

[martin-mat]

Currently, it is possible to run cnf_setup multiple times for multiple CNFs/helm charts. When running tests then, tests are executed on each configured CNFs. Clarify whether this is a supported feature. Currently, there is no documentation nor tests of this behavior. If this should be supported behavior, address those aspects:

1. Add this to documentation
2. Add spec test(s) to verify expected functionality
3. Fix behavior of generating result yaml files. Currnently, just one yaml file is being generated; results of particular test are overwritten, and it records result only of the latest CNF. Which if obviously not expected behavior. Right behavior needs to be figured out and implemented.

If this is not supported usage, don't allow to call cnf_setup multiple times.