rpc: Optimize serialization disk space of dumptxoutset

Co-authored-by: Aurèle Oulès <aurele@oules.com>
Co-authored-by: TheCharlatan <seb.kung@gmail.com>

src/test/validation_chainstatemanager_tests.cpp

@@ -226,10 +226,13 @@ struct SnapshotTestSetup : TestChain100Setup {
                 // A UTXO is missing but count is correct
                 metadata.m_coins_count -= 1;
 
-                COutPoint outpoint;
+                Txid txid;
+                auto_infile >> txid;
+                // coins size
+                (void)ReadCompactSize(auto_infile);
+                // vout index
+                (void)ReadCompactSize(auto_infile);
                 Coin coin;
-
-                auto_infile >> outpoint;
                 auto_infile >> coin;
         }));