Bitcoind: Need to explicitly create wallet for tests

[pavitthrap]

As of bitcoin 0.21.0, a default wallet is no longer automatically created (bitcoin/bitcoin#15454).

This change causes the tests in neon_integrations.rs to fail with the following message:

ERRO [1617206434.490494] [testnet/stacks-node/src/burnchains/bitcoin_regtest_controller.rs:589] [ThreadId(13)] Bitcoin RPC failure: error listing utxos Network("Bitcoin RPC: status(500) != success, body is \'Object({\"error\": Object({\"code\": Number(-18), \"message\": String(\"No wallet is loaded. Load a wallet using loadwallet or create a new one with createwallet. (Note: A default wallet is no longer automatically created)\")}), \"id\": String(\"stacks\"), \"result\": Null})\'")

[pavitthrap]

Fixed in #2581

[blockstack-devops]

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