When you test a plugin in qunit, we only load the JS for that specific plugin. That helps to avoid surprise issues when multiple plugins interact with each other. It also helps to ensure that a plugin doesn’t unexpectedly become dependent on another.
In your case, it sounds like the interaction is deliberate, and you always expect your users to have discourse-calendar enabled. In that case, you can add some config to an about.json file which will make the discourse-calendar JS available in the tests of your own plugin. Here’s an example:
^^ in this case it’s adding discourse-assign. For your case, you would make it discourse-calendar