Introduce DummyDatabase and use it in the tests

src/wallet/walletdb.cpp

@@ -1021,7 +1021,7 @@ std::unique_ptr<WalletDatabase> CreateWalletDatabase(const fs::path& path)
 /** Return object for accessing dummy database with no read/write capabilities. */
 std::unique_ptr<WalletDatabase> CreateDummyWalletDatabase()
 {
-    return MakeUnique<BerkeleyDatabase>();
+    return MakeUnique<DummyDatabase>();
 }
 
 /** Return object for accessing temporary in-memory database. */