[avalanche] split abc_p2p_avalanche into 2 tests

Summary:
This splits the abc_p2p_avalanche functional test suite into two smaller and easier to maintain suites:

• abc_p2p_avalanche_voting
• abc_p2p_avalanche_peer_discovery

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9685