wallet, tests: Include wallet/test/util.h

This will be needed for the following scripted-diff to work.

src/bench/wallet_loading.cpp

@@ -17,6 +17,7 @@
 #include <optional>
 
 using wallet::CWallet;
+using wallet::CreateMockableWalletDatabase;
 using wallet::DatabaseFormat;
 using wallet::DatabaseOptions;
 using wallet::TxStateInactive;