Computational fluid dynamics (CFD, of which climate modeling is a subfield) actually does software testing better than other scientific fields in my experience. That's not to say that all CFD models are well tested, just that they're doing a fair bit more than nothing. Typically testing is divided into "verification", which is checking the math, and "validation" which is comparing against experimental data. If you can't find anything about a model's verification and validation, don't trust it.
Also, it's actually fairly easy to get apparently good validation by cherry picking the right data, so even doing well here isn't enough. You basically need to be an expert to know the right test cases to look at...
Also, it's actually fairly easy to get apparently good validation by cherry picking the right data, so even doing well here isn't enough. You basically need to be an expert to know the right test cases to look at...
Here's a slide deck that shows the (not great) state of typical practice in validation: https://www.osti.gov/biblio/1141709
Note that some of the "better" journals do worse...
Here's a public domain CFD software that I think does a good job on verification and validation: https://github.com/firemodels/fds
Full disclosure: I have contributed to this project.