Don't create another wallet db directory in walletdb_tests
Don't create another wallet db directory in walletdb_tests

Summary:
The `walletdb_tests` test cases start by creating a wallet db file in a
new directory. Now that the wallet test fixture is used (D3099), this
can be avoided by using the test fixture `pwalletMain` db instead.

Test Plan:
  ./src/test/test_bitcoin -t walletdb_tests

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3102

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