Not sure how the tests are run, I don’t see any CI setup in the repo.
You’re probably right, if you want to do anything useful you’ll likely need to follow the basic instructions on getting started with a C/C++ project: Mongoose OS Documentation
I’m guessing you’ll want to clone the c-demo app, not the js app. If you wanted to test the actual cron library on your device, then add the cron library to the mos.yml file in your new project.
I don’t use the cloud compilation service, I build locally which pulls the source files required to build - I’m guessing are required to run the tests based on your comment. If you get stuck there because source files are not being pulled, take a look at mos build locally: the mos build flag you want is --local, but docker is required. You might find this thread handy.
There are a few threads on this forum about speeding things up, but cross that bridge when you get to it.