[V132-specific] Only spend UTXO unused gas for Migrate outbounds

[V132-specific]

Intended to close #1878 (closed)
'Commit e8fe6be0 appears to avoidably send unused gas to UTXO outbound recipients'.

(This only affects the consolidated UTXO client code as the chain-specific client code has conveniently already been cleared in !3378 (merged).)

(Please note that the TxMigrate check doesn't need a simultaneous TxConsolidate check as that already exists just below;
it assumes MaxGas to be empty for TxConsolidate, unlike for TxMigrate.)

bifrost/pkg/chainclients/utxo/bitcoin_signer_test.go

@@ -30,6 +30,7 @@ import (
 	"gitlab.com/thorchain/thornode/common"
 	"gitlab.com/thorchain/thornode/common/cosmos"
 	"gitlab.com/thorchain/thornode/config"
+	"gitlab.com/thorchain/thornode/x/thorchain"
 	types2 "gitlab.com/thorchain/thornode/x/thorchain/types"
 )
 
@@ -205,6 +206,9 @@ func (s *BitcoinSignerSuite) TestSignTx(c *C) {
 func (s *BitcoinSignerSuite) TestSignTxHappyPathWithPrivateKey(c *C) {
 	addr, err := types2.GetRandomPubKey().GetAddress(common.BTCChain)
 	c.Assert(err, IsNil)
+	inHash := thorchain.GetRandomTxHash()
+	memo := "OUT:" + inHash.String() // Memo must be parsable or ParseMemo will error.
+
 	txOutItem := stypes.TxOutItem{
 		Chain:       common.BTCChain,
 		ToAddress:   addr,
@@ -215,8 +219,9 @@ func (s *BitcoinSignerSuite) TestSignTxHappyPathWithPrivateKey(c *C) {
 		MaxGas: common.Gas{
 			common.NewCoin(common.BTCAsset, cosmos.NewUint(1000)),
 		},
-		InHash:  "",
+		InHash:  inHash,
 		OutHash: "",
+		Memo:    memo,
 	}
 	txHash := "256222fb25a9950479bb26049a2c00e75b89abbb7f0cf646c623b93e942c4c34"
 	c.Assert(err, IsNil)