multi: migrate instances of mockSigner to the mock package

This commit moves all localized instances of mock implementations of
the Signer interface to the lntest/mock package. This allows us to
remove a lot of code and have it housed under a single interface in
many cases.

sweep/test_utils.go

@@ -6,11 +6,9 @@ import (
 	"testing"
 	"time"
 
-	"github.com/btcsuite/btcd/btcec"
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/wire"
 	"github.com/lightningnetwork/lnd/chainntnfs"
-	"github.com/lightningnetwork/lnd/input"
 )
 
 var (
@@ -19,31 +17,6 @@ var (
 	mockChainHeight    = int32(100)
 )
 
-type dummySignature struct{}
-
-func (s *dummySignature) Serialize() []byte {
-	return []byte{}
-}
-
-func (s *dummySignature) Verify(_ []byte, _ *btcec.PublicKey) bool {
-	return true
-}
-
-type mockSigner struct {
-}
-
-func (m *mockSigner) SignOutputRaw(tx *wire.MsgTx,
-	signDesc *input.SignDescriptor) (input.Signature, error) {
-
-	return &dummySignature{}, nil
-}
-
-func (m *mockSigner) ComputeInputScript(tx *wire.MsgTx,
-	signDesc *input.SignDescriptor) (*input.Script, error) {
-
-	return &input.Script{}, nil
-}
-
 // MockNotifier simulates the chain notifier for test purposes. This type is
 // exported because it is used in nursery tests.
 type MockNotifier struct {