Add unit tests

[mumoshu]

I took a brief look on the current code-base, and noticed that there're no unit tests!

Adding ones would help us better maintain this project in longer term.

Also, I'm happy to contribute here.
Please let me know if there's specific type/func to which you'd like to add tests.

[khash]

I think starting at the build flow level is a good one by adding unit tests around a single step when artifacts are pulled out as well as tests around secret providers.