test: Increase test coverage

[kurosakishigure]

Test Description

Increase the scope of unit tests. Missing test cases can be found on Coveralls.

Rationale

Test coverage is low.

Additional Information

We can refer to the code marked as lacking tests on Coveralls and add corresponding test cases.

[kurosakishigure]

The file that has the greatest impact on coverage is handle.rs.

The key lines are:

katharsis/src/cmd/handle.rs

Line 46 in 215689f

let ans: &str = Select::new("A katharsis.config.toml already exists in the current directory. Do you want to overwrite it?", options).prompt()?;

When testing, we should think about how to simulate user input🤔.

Tips: We are considering adding a -y parameter.