Add CIME unit tests for behavior relied on in CTSM

From [this discussion](https://docs.google.com/document/d/1tgDRocjAFF1svWjt0mvjjPaPLFuDZEHLpf4vlGIQs5o/edit#heading=h.84gnl0ym5u5) from today's CTSM SE meeting:

We need to add unit tests to ensure that CIME behaves as we expect it to. As a starting point, we can copy any existing unit tests from CIME to our own repo.