Adding missed unit test cases

[CXWorks]

Hi,

Thanks for your time & patience to review this PR.

We are researchers focusing on Rust unit tests. By examine the existing code, we found a unit test can be added to improve the repo's overall unit test coverage(this project is already been well tested). For the first 4 cases, you can easily find their error locations and for the last case, the untested area is: https://github.com/tailhook/humantime/blob/12ce6f50894a56a410b390e5608ac9db8afe2407/src/duration.rs#L181-L187 including both the error cases and correct path.

Thanks again for reviewing.