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.

htlcswitch/test_utils.go

@@ -29,6 +29,7 @@ import (
 	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/keychain"
 	"github.com/lightningnetwork/lnd/lnpeer"
+	"github.com/lightningnetwork/lnd/lntest/mock"
 	"github.com/lightningnetwork/lnd/lntest/wait"
 	"github.com/lightningnetwork/lnd/lntypes"
 	"github.com/lightningnetwork/lnd/lnwallet"
@@ -376,8 +377,8 @@ func createTestChannel(alicePrivKey, bobPrivKey []byte,
 		os.RemoveAll(alicePath)
 	}
 
-	aliceSigner := &mockSigner{aliceKeyPriv}
-	bobSigner := &mockSigner{bobKeyPriv}
+	aliceSigner := &mock.SingleSigner{Privkey: aliceKeyPriv}
+	bobSigner := &mock.SingleSigner{Privkey: bobKeyPriv}
 
 	alicePool := lnwallet.NewSigPool(runtime.NumCPU(), aliceSigner)
 	channelAlice, err := lnwallet.NewLightningChannel(