[avalanche] Test: move the AvalancheTestingSetup to the common test header

Summary:
This makes it possible to reuse for tests that need avalanche. There is no change in behavior.

Depends on D18308.

Test Plan:

ninja check

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18310