itest: run normal FundPsbt test case in remote-signer mode

This makes sure there is no general issue with running the pay-join
FundPsbt test case in a remote signer setup.

itest/lnd_psbt_test.go

@@ -1060,6 +1060,14 @@ func testFundPsbt(ht *lntest.HarnessTest) {
 	alice := ht.NewNodeWithCoins("Alice", nil)
 	bob := ht.NewNodeWithCoins("Bob", nil)
 
+	runFundPsbt(ht, alice, bob)
+}
+
+// runFundPsbt tests the FundPsbt RPC use case where we want to fund a PSBT
+// that already has an input specified. This is a pay-join scenario where Bob
+// wants to send Alice some coins, but he wants to do so in a way that doesn't
+// reveal the full amount he is sending.
+func runFundPsbt(ht *lntest.HarnessTest, alice, bob *node.HarnessNode) {
 	// We test a pay-join between Alice and Bob. Bob wants to send Alice
 	// 5 million Satoshis in a non-obvious way. So Bob selects a UTXO that's
 	// bigger than 5 million Satoshis and expects the change minus the send

itest/lnd_remote_signer_test.go

@@ -328,6 +328,11 @@ func testRemoteSignerPSBT(ht *lntest.HarnessTest) {
 			// that aren't in the wallet. But we also want to make
 			// sure we can fund and then sign PSBTs from our wallet.
 			runFundAndSignPsbt(ht, wo)
+
+			// We also have a more specific funding test that does
+			// a pay-join payment with Carol.
+			ht.FundCoins(btcutil.SatoshiPerBitcoin, carol)
+			runFundPsbt(ht, wo, carol)
 		},
 	}