rpcwallet+itest: fix incomplete key info problem

Fixes an issue with SignOutputRaw in remote signing mode where we
weren't able to sign on the remote signer if we only provided the public
key or only the family/index (and not both).

Fixes part of an issue detected in lightninglabs/loop#457.

lntest/itest/lnd_remote_signer_test.go

@@ -99,6 +99,12 @@ func testRemoteSigner(net *lntest.NetworkHarness, t *harnessTest) {
 			runPsbtChanFunding(net, tt, carol, wo)
 			runSignPsbt(tt, net, wo)
 		},
+	}, {
+		name:      "sign output raw",
+		sendCoins: true,
+		fn: func(tt *harnessTest, wo, carol *lntest.HarnessNode) {
+			runSignOutputRaw(tt, net, wo)
+		},
 	}}
 
 	for _, st := range subTests {