kad: Randomize node count in get_providers_limit test

[GarmashAlex]

Description

Resolves the TODO in the get_providers_limit test by replacing the fixed number
of nodes (12) with a randomized value between 3 and 20. This makes the test more
robust by testing with different network sizes on each run.

Also fixes a type inference issue with the const generic parameter N by explicitly
specifying the type when passing the prop function to QuickCheck.

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates

[elenaf9]

The test only uses 3 nodes, it doesn't make any sense to create more if they don't participate.