测试案例设计：确保案例独立性

When design test cases, try not have cases which have internal dependancies to eachother. The more independant, the better.

This is because if cases are depending to each other:

1. Propagate failures may be a lot  if  test cases  previously executed fails,  it is difficult(or even impossible)  to isolate key failures from propagated failures.
2. Propagate failures may override/hide other key failures, and reduce the effectiveness of a test case.
3. some propagate failure may be taken as a key failure in some cases, -- fault failures are collected.