lnwallet/btcwallet: add compile time check for BlockChainIO interface

mock.go

@@ -205,6 +205,8 @@ type mockChainIO struct {
 	bestHeight int32
 }
 
+var _ lnwallet.BlockChainIO = (*mockChainIO)(nil)
+
 func (m *mockChainIO) GetBestBlock() (*chainhash.Hash, int32, error) {
 	return activeNetParams.GenesisHash, m.bestHeight, nil
 }