Add a unit test suite

[doug-newman-nasa]

A unit test suite would increase the confidence of any code contributions from the community. It is also something that NASA Earthdata may provide via an intern project this summer. Writing the issue now as we may provide some earlier contributions to this effort.

[m-mohr]

Yeah, we had so many regressions in the past, it's definitively time to do it and should be worth the effort. I's high on my priority list as well, but I certainly welcome contributions! If you want to talk about how we can achieve this, e.g. with the help of an intern, I'm happy to talk. Having that said, we also face the Vue 3 and OL migration, which will also lead to a v4 of STAC Browser in the next year or so.