wallet: move Assert() check into constructor

This puts it in a function body, so that __func__ is available
for reporting any assertion failure.

src/wallet/test/wallet_test_fixture.cpp

@@ -9,6 +9,7 @@
 namespace wallet {
 WalletTestingSetup::WalletTestingSetup(const std::string& chainName)
     : TestingSetup(chainName),
+      m_wallet_loader{interfaces::MakeWalletLoader(*m_node.chain, *Assert(m_node.args))},
       m_wallet(m_node.chain.get(), "", m_args, CreateMockWalletDatabase())
 {
     m_wallet.LoadWallet();