sweep: add wallet inputs to reach dust limit

This commit allows sweeper to sweep inputs that on its own are not able
to form a sweep transaction that meets the dust limit.

This functionality is useful for sweeping small outputs. In the future,
this will be particularly important to sweep anchors. Anchors will
typically be spent with a relatively large fee to pay for the parent tx.
It will then be necessary to attach an additional wallet utxo.

sweep/backend_mock_test.go

@@ -25,6 +25,8 @@ type mockBackend struct {
 	unconfirmedSpendInputs map[wire.OutPoint]struct{}
 
 	publishChan chan wire.MsgTx
+
+	walletUtxos []*lnwallet.Utxo
 }
 
 func newMockBackend(t *testing.T, notifier *MockNotifier) *mockBackend {
@@ -84,6 +86,16 @@ func (b *mockBackend) PublishTransaction(tx *wire.MsgTx) error {
 	return err
 }
 
+func (b *mockBackend) ListUnspentWitness(minconfirms, maxconfirms int32) (
+	[]*lnwallet.Utxo, error) {
+
+	return b.walletUtxos, nil
+}
+
+func (b *mockBackend) WithCoinSelectLock(f func() error) error {
+	return f()
+}
+
 func (b *mockBackend) deleteUnconfirmed(txHash chainhash.Hash) {
 	b.lock.Lock()
 	defer b.lock.Unlock()