[avalanche] Give peer discovery functional test class a unique name

Summary: At best having the same class name could be confusing while debugging. At worst this could break something in unexpected ways.

Test Plan:

ninja
./test/functional/test_runner.py abc_p2p_avalanche_peer_discovery

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12472