aelf-web3.js - Improve unit test cases

### Expected Behavior
Improve unit test cases of aelf-web3.js

### 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

**Development Tasks:**
- [ ] TBD

### Dependencies
nil

### References
nil