input+wallet: extract musig2 session management into new module

In this commit, we extract the musig2 session management into a new
module. This allows us to re-use the session logic elsewhere in unit
tests so we don't need to instantiate the entire wallet.

htlcswitch/test_utils.go

@@ -32,7 +32,6 @@ import (
 	"github.com/lightningnetwork/lnd/kvdb"
 	"github.com/lightningnetwork/lnd/lnpeer"
 	"github.com/lightningnetwork/lnd/lntest/channels"
-	"github.com/lightningnetwork/lnd/lntest/mock"
 	"github.com/lightningnetwork/lnd/lntest/wait"
 	"github.com/lightningnetwork/lnd/lntypes"
 	"github.com/lightningnetwork/lnd/lnwallet"
@@ -337,8 +336,12 @@ func createTestChannel(t *testing.T, alicePrivKey, bobPrivKey []byte,
 		return nil, nil, err
 	}
 
-	aliceSigner := &mock.SingleSigner{Privkey: aliceKeyPriv}
-	bobSigner := &mock.SingleSigner{Privkey: bobKeyPriv}
+	aliceSigner := input.NewMockSigner(
+		[]*btcec.PrivateKey{aliceKeyPriv}, nil,
+	)
+	bobSigner := input.NewMockSigner(
+		[]*btcec.PrivateKey{bobKeyPriv}, nil,
+	)
 
 	alicePool := lnwallet.NewSigPool(runtime.NumCPU(), aliceSigner)
 	channelAlice, err := lnwallet.NewLightningChannel(