rpc, tests: in utxoupdatepsbt also look for the transaction in the txindex

Previously only the segwit utxos being spent by the psbt were looked for and
added to the psbt. Now, the full transaction corresponding to each of these
utxos (legacy and segwit) is looked for in the txindex and mempool and added
to the psbt. If txindex is disabled and the transaction is not in the mempool,
then we fall back to getting just the utxo (if segwit) from the utxo set.

src/psbt.cpp

@@ -430,6 +430,38 @@ bool SignPSBTInput(const SigningProvider& provider, PartiallySignedTransaction&
     return sig_complete;
 }
 
+void RemoveUnnecessaryTransactions(PartiallySignedTransaction& psbtx, const int& sighash_type)
+{
+    // Only drop non_witness_utxos if sighash_type != SIGHASH_ANYONECANPAY
+    if ((sighash_type & 0x80) != SIGHASH_ANYONECANPAY) {
+        // Figure out if any non_witness_utxos should be dropped
+        std::vector<unsigned int> to_drop;
+        for (unsigned int i = 0; i < psbtx.inputs.size(); ++i) {
+            const auto& input = psbtx.inputs.at(i);
+            int wit_ver;
+            std::vector<unsigned char> wit_prog;
+            if (input.witness_utxo.IsNull() || !input.witness_utxo.scriptPubKey.IsWitnessProgram(wit_ver, wit_prog)) {
+                // There's a non-segwit input or Segwit v0, so we cannot drop any witness_utxos
+                to_drop.clear();
+                break;
+            }
+            if (wit_ver == 0) {
+                // Segwit v0, so we cannot drop any non_witness_utxos
+                to_drop.clear();
+                break;
+            }
+            if (input.non_witness_utxo) {
+                to_drop.push_back(i);
+            }
+        }
+
+        // Drop the non_witness_utxos that we can drop
+        for (unsigned int i : to_drop) {
+            psbtx.inputs.at(i).non_witness_utxo = nullptr;
+        }
+    }
+}
+
 bool FinalizePSBT(PartiallySignedTransaction& psbtx)
 {
     // Finalize input signatures -- in case we have partial signatures that add up to a complete