[avalanche] Generate a single block in tests

Summary: Generating 2 blocks is not needed anymore.

Test Plan:

ninja check-functional
./test/functional/test_runner.py p2p_inv_download

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9696