test: improve test coverage

[NozomuIkuta]

Describe the feature

I found that Citty has no test at the moment.
It's better to have at least unit tests so that we can accept new features and bug fixes from the community as well as unjs members.

I created this issue just for making the situation clear and trackable.

@pi0

Perhaps, are you postponing test implementation based one some thoughts?
If not, I will start working on this issue from adding unit tests.

Additional information

• Would you be willing to help implement this feature?