Implemented a block signature verification unit test.
Implemented a block signature verification unit test.

Testing:

1) P2PK block signature pre v5 enforcement.
2) P2PK block signature post v5 enforcement.
3) P2PKH block signature pre v5 enforcement.
4) P2PKH block signature post v5 enforcement.

PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,Darknet-Crypto/Darknet,PIVX-Project/PIVX,Darknet-Crypto/Darknet,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,Darknet-Crypto/Darknet,Darknet-Crypto/Darknet,Darknet-Crypto/Darknet,Darknet-Crypto/Darknet