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