Use a different address in Avalanche test to prevent duplicate blocks

Summary:
regtest blocks are relatively deterministic, so generating a block with the
same height, coinbase, timestamp, etc. may coincide with a block that was already generated.
In this case, it's possible to coincide with a block that was invalidated using the same
coinbase address above this line.

Test Plan:

test_runner.py abc-p2p-avalanche

Run this many times. On master, it's encountered ~10% of the time.
With this patch, we should not encounter it.

Note that abc-p2p-avalanche is still broken on master.

Reviewers: #bitcoin_abc, Fabien, deadalnix

Reviewed By: #bitcoin_abc, Fabien, deadalnix

Subscribers: deadalnix, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5691