Add AssertLockHeld assertions in CWallet::ListCoins

Also update wallet tests to acquire the cs_main lock correctly when calling
ListCoins, and to release the cs_wallet lock before calling
ScanForWalletTransactions (because ScanForWalletTransactions acquired cs_main
internally, which should never be acquired after cs_wallet because that would
trigger anti-deadlock warnings).

Author: ryanofsky

src/wallet/wallet.cpp

@@ -2193,22 +2193,14 @@ void CWallet::AvailableCoins(std::vector<COutput> &vCoins, bool fOnlySafe, const
 
 std::map<CTxDestination, std::vector<COutput>> CWallet::ListCoins() const
 {
-    // TODO: Add AssertLockHeld(cs_wallet) here.
-    //
-    // Because the return value from this function contains pointers to
-    // CWalletTx objects, callers to this function really should acquire the
-    // cs_wallet lock before calling it. However, the current caller doesn't
-    // acquire this lock yet. There was an attempt to add the missing lock in
-    // https://github.com/bitcoin/bitcoin/pull/10340, but that change has been
-    // postponed until after https://github.com/bitcoin/bitcoin/pull/10244 to
-    // avoid adding some extra complexity to the Qt code.
+    AssertLockHeld(cs_main);
+    AssertLockHeld(cs_wallet);
 
     std::map<CTxDestination, std::vector<COutput>> result;
 
     std::vector<COutput> availableCoins;
     AvailableCoins(availableCoins);
 
-    LOCK2(cs_main, cs_wallet);
     for (auto& coin : availableCoins) {
         CTxDestination address;
         if (coin.fSpendable &&