[avalanche] Templatize the AvalancheTestingSetup

Summary:
This makes it possible to use avalanche on any BasicTestingSetup child class. This will become useful when updating the mining tests for preconsensus.
The final structure is renamed appropriately.

There is no change in behavior.

Test Plan:

ninja all check

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18326