Update testnet difficulty cap to 3,000,000,000

This is the new difficulty cap value since HSM v5.4.1, see rsksmart/rsk-powhsm#300

Author: marcos-iov

src/main/java/co/rsk/federate/signing/hsm/config/NetworkDifficultyCap.java

@@ -4,7 +4,7 @@
 
 public enum NetworkDifficultyCap {
   MAINNET(new BigInteger("7000000000000000000000")),
-  TESTNET(BigInteger.valueOf(1000000000000000L)),
+  TESTNET(BigInteger.valueOf(3000000000L)),
   REGTEST(BigInteger.valueOf(20L));
 
   private final BigInteger difficultyCap;