Add system test: Creating single-point run with NUOPC

[Apparently](https://github.com/ESCOMP/CTSM/discussions/1833#discussioncomment-11329245) something has changed that makes the "[How do I create a single-point run with the NUOPC (default) coupler?" instructions](https://github.com/ESCOMP/CTSM/discussions/1833) not work anymore.

I've filed an issue about fixing that as #2890. In _this_ issue, I'm proposing we set up a system test for this so that we don't accidentally break things again.