Feature : Code coverage reaches 60%

[LucasMLK]

For the stability of future version releases, code coverage needs to be completed, with a target of 60%

1.Use maven plugin to count existing unit test code coverage
2.Implement core code unit test coverage

• [io.xdag.cli ]
• [io.xdag.config ]
• [io.xdag.consensus ]
• [io.xdag.core ]
• [io.xdag.crypto ]
• [io.xdag.mine ]
• [io.xdag.wallet ]

3. Achieving the goal of 60% of all code coverage