message: 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.
diff: @@ -148,12 +148,9 @@ 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());
  