aelf-sdk.cs - Improve unit test cases for IClientService

[zhifenglee-aelf]

Expected Behavior
Improve unit test cases of aelf-sdk.cs for IClientService

Specifications
As a developer, I would like to have an extensive unit test cases so that code can be checked for regression when ever there is a code change.

Features:

• Negative test cases
• Failed test cases

A document outlining the structure of aelf Documentation website.
Dependencies
nil

References
nil