Add AppRequestNetwork unit tests

[cam-schultz]

Context and scope
AppRequestNetwork is a wrapper around the AvalancheGo TestNetwork construct, which facilitates non-node peer-to-peer connections with validators. The wrapper has gotten more complex, as we've added support for earlier peer validation and manual peer connection. We should add unit tests to support this type.

Discussion and alternatives
In order to write unit tests, we'll need mocks for:

• network.Network
• platformvm.Client
• info.Client

Open questions
What's the best way to mock interfaces from external packages? An AvalancheGo PR may be necessary.

[najeal]

A good tool to mock interfaces from external packages: https://vektra.github.io/mockery/latest/