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:

humantime/src/duration.rs

Lines 181 to 187 in 12ce6f5

	'0'..='9' => {
	self.parse_unit(n, start, off)?;
	n = c as u64 - '0' as u64;
	continue 'outer;
	}
	c if c.is_whitespace() => break,
	'a'..='z' | 'A'..='Z' => {}

including both the error cases and correct path.

Thanks again for reviewing.