Snap values for discrete parameters

[ffAudio]

When setting random values to discrete parameters, they will be snapped to the nearest legitimate value.
This will subesquently lead to false positives when comparing the value that was read back.

This PR makes sure, that the test get only legit values set for discrete parameters, especially boolean and choice parameters.

[magnetophon]

Any progress on this?
I've read various threads on the juce form, such as https://forum.juce.com/t/pluginval-clarification/65693 and https://forum.juce.com/t/pluginval-can-set-an-audioparameterbools-to-odd-values/50971
I don't have an account on that forum, so I'm asking here.