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

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

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
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