tests: add unit tests for all files

[rohansen856]

Describe your improvement idea or feature request

A new version of taskwarrior can be released, but we need to ensure all functionalities of the app is working fine. Writing unit tests for all the files would be the best way to go forward.

How would you like to see it implemented?

write tests for all the files under tests/ directory

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Yes

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[rohansen856]

As the taskwarrior repo is quite big, It would need sub issues to correctly test all the code (folderwise perhaps). @Pavel401 Please assign me this issue.