Always run a "common denominator" cmd/cue command on all Unity projects

[mvdan]

Some projects integrated with Unity declare their own cue.mod/tests/, and for some others we write the tests directory in our overlay. In either case, we should always run a basic command to give us some minimum confidence that all the CUE files in the project evaluate in the same way. This might be cue eval ./..., or it might be some other command that would do a better job at "the semantics are unchanged".

cc @myitcv

[myitcv]

cc @mpvl.

For now we use cue eval for this purpose. This covers the fact that we cannot know whether the configurations ./... should be concrete or not.

@mvdan and I noted this in conversations about adding projects to unity where the project owner does not specify any tests/assertions. In this case, not only is this "common denominator" mode required, but it needs to work with cue diff.