Refactor walletdb_tests to use the wallet test fixture

Summary:
The src/wallet/test/walletdb_tests.cpp test file defines its own test
fixture, which is equivalent to a subset of the WalletTestingSetup
from src/wallet/test/wallet_test_fixture.h.
Using the WalletTestingSetup will reduce maintenance cost.

Test Plan:

make check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3099