Fix Avalanche functional test when wallet is disabled
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

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