Move unittests to one central location

[christinaholtNOAA]

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Description

The unittests are scattered among a bunch of files throughout the ush directory. Moving them to a centralized location will give us an easier way to make sure they all run, and will help encourage developers to run them. We should also have them run as part of our Jenkins pipeline, so simplifying how to run them will be helpful there, too.

The unittests run on front-end nodes so should not take up any additional allocation on-prem.

Solution

Move all existing unittests to a centralized location under tests

This was originally proposed in Oct 2022 by (I believe) @gsketefian in a SRW Dev Meeting.