Add tests for incorrect error messages of association length validations

Hey :)
These are tests showing issue #16735 in detail

As @s-s-w mentioned, the problem is with validating length of associations. Error messages of length validators are composed as if they were supposed to be used with strings only.

We can change error messages to be type-agnostic, for example 
```"is too short (minimum length is 2)"``` 
instead of
```"is too short (minimum is 2 characters)"```
or check what exactly do we validate and decide based on that. Any other ideas?