Fix Avalanche functional test when wallet is disabled

Summary: Similar to D5568, generate() doesn't work when the wallet is disabled.

Test Plan:

cmake -GNinja -DBUILD_BITCOIN_WALLET=OFF ..
ninja
./test/functional/test_runner.py abc-p2p-avalanche

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5685