Refactor walletdb_tests to use the wallet test fixture
The src/wallet/test/walletdb_tests.cpp test file defines its own test
fixture, which is equivalent to a subset of the WalletTestingSetup
Using the WalletTestingSetup will reduce maintenance cost.
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D3099