test: use MiniWallet for getblockstats test data generation #35177

Fixes #31838. Follow-up to #33184.

This updates rpc_getblockstats.py --gen-test-data to generate its fixture data using MiniWallet instead of wallet RPCs.

The normal test path still loads predefined block/stat data from rpc_getblockstats.json, so the test continues to compare getblockstats results against fixed expected data rather than against freshly computed RPC output.

This keeps the generator independent of wallet behavior while preserving the existing regression-test structure.

Tested on a wallet-disabled Linux build:

• python3 /tmp/bitcoin-build/test/functional/rpc_getblockstats.py
• python3 /tmp/bitcoin-build/test/functional/rpc_getblockstats.py --gen-test-data --test-data /tmp/rpc_getblockstats.generated.json
• python3 /tmp/bitcoin-build/test/functional/rpc_getblockstats.py --test-data /tmp/rpc_getblockstats.generated.json

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In the pull description: You can start the test from the build dir, to pick up the ini automatically. That is python3 test/functional/rpc_getblockstats.py --configfile=/tmp/bitcoin-build/test/config.ini becomes python3 /tmp/bitcoin-build/test/functional/rpc_getblockstats.py etc.

In the pull description: You can start the test from the build dir, to pick up the ini automatically. That is python3 test/functional/rpc_getblockstats.py --configfile=/tmp/bitcoin-build/test/config.ini becomes python3 /tmp/bitcoin-build/test/functional/rpc_getblockstats.py etc.

Updated the test commands in the PR description, thanks.