wallet refactor: Avoid use of Chain::Lock in qt wallettests

This is a step toward removing the Chain::Lock class and reducing cs_main
locking.

This change doesn't affect behavior.

src/qt/test/wallettests.cpp

@@ -151,12 +151,9 @@ void TestGUI(interfaces::Node& node)
         wallet->SetLastBlockProcessed(105, ::ChainActive().Tip()->GetBlockHash());
     }
     {
-        auto locked_chain = wallet->chain().lock();
-        LockAssertion lock(::cs_main);
-
         WalletRescanReserver reserver(wallet.get());
         reserver.reserve();
-        CWallet::ScanResult result = wallet->ScanForWalletTransactions(locked_chain->getBlockHash(0), {} /* stop_block */, reserver, true /* fUpdate */);
+        CWallet::ScanResult result = wallet->ScanForWalletTransactions(Params().GetConsensus().hashGenesisBlock, {} /* stop_block */, reserver, true /* fUpdate */);
         QCOMPARE(result.status, CWallet::ScanResult::SUCCESS);
         QCOMPARE(result.last_scanned_block, ::ChainActive().Tip()->GetBlockHash());
         QVERIFY(result.last_failed_block.IsNull());