tests: Test specifying input weights

Added tests to rpc_fundrawtransaction, wallet_send, and rpc_psbt that
test that external inputs can be spent when input weight is provided.
Also tested that the input weight overrides any calculated weight.

Additionally, rpc_psbt's external inputs test is cleaned up a bit to be
more similar to rpc_fundrawtransaction's and avoid potential pitfalls
due to non-deterministic coin selection behavior.

test/functional/wallet_send.py

@@ -518,5 +518,45 @@ class WalletSendTest(BitcoinTestFramework):
         assert signed["complete"]
         self.nodes[0].finalizepsbt(signed["psbt"])
 
+        dec = self.nodes[0].decodepsbt(signed["psbt"])
+        for i, txin in enumerate(dec["tx"]["vin"]):
+            if txin["txid"] == ext_utxo["txid"] and txin["vout"] == ext_utxo["vout"]:
+                input_idx = i
+                break
+        psbt_in = dec["inputs"][input_idx]
+        # Calculate the input weight
+        # (prevout + sequence + length of scriptSig + 2 bytes buffer) * 4 + len of scriptwitness
+        len_scriptsig = len(psbt_in["final_scriptSig"]["hex"]) // 2 if "final_scriptSig" in psbt_in else 0
+        len_scriptwitness = len(psbt_in["final_scriptwitness"]["hex"]) // 2 if "final_scriptwitness" in psbt_in else 0
+        input_weight = ((41 + len_scriptsig + 2) * 4) + len_scriptwitness
+
+        # Input weight error conditions
+        assert_raises_rpc_error(
+            -8,
+            "Input weights should be specified in inputs rather than in options.",
+            ext_wallet.send,
+            outputs={self.nodes[0].getnewaddress(): 15},
+            options={"inputs": [ext_utxo], "input_weights": [{"txid": ext_utxo["txid"], "vout": ext_utxo["vout"], "weight": 1000}]}
+        )
+
+        # Funding should also work when input weights are provided
+        res = self.test_send(
+            from_wallet=ext_wallet,
+            to_wallet=self.nodes[0],
+            amount=15,
+            inputs=[{"txid": ext_utxo["txid"], "vout": ext_utxo["vout"], "weight": input_weight}],
+            add_inputs=True,
+            psbt=True,
+            include_watching=True,
+            fee_rate=10
+        )
+        signed = ext_wallet.walletprocesspsbt(res["psbt"])
+        signed = ext_fund.walletprocesspsbt(res["psbt"])
+        assert signed["complete"]
+        tx = self.nodes[0].finalizepsbt(signed["psbt"])
+        testres = self.nodes[0].testmempoolaccept([tx["hex"]])[0]
+        assert_equal(testres["allowed"], True)
+        assert_fee_amount(testres["fees"]["base"], testres["vsize"], Decimal(0.0001))
+
 if __name__ == '__main__':
     WalletSendTest().main()