message: fix(statics): inherit BitcoinTestnet from Testnet
  Issue:
diff: @@ -101,7 +101,7 @@ class Bitcoin extends Mainnet implements UtxoNetwork {
  explorerUrl = 'https://blockstream.info/tx/';
  }
  -class BitcoinTestnet extends Mainnet implements UtxoNetwork {
  +class BitcoinTestnet extends Testnet implements UtxoNetwork {
  name = 'BitcoinTestnet';
  family = CoinFamily.BTC;
  utxolibName = 'testnet';
  