Super Thanks, Graou74!
The jar you uploaded does check out with my 11 and 13 runtimes as having a valid module descriptor 
However, given all of the other evidence considered as a whole (the debug output, the results of my experiments in Linux, etc) my certainty approaches 100% that your local java.lang.module.InvalidModuleDescriptorException: failure is caused by your local instance of myApplication.jar having been corrupted — somehow.
An InvalidModuleDescriptorException can be caused by several different things. My hunch — based on my recent Googling and experiments — is that one way or another the project’s jar was compiled by javac with a high value for --release (13, say). But then the java runtime that ran the :test task was lower than that value (11, say).
So what do I propose as that real conclusive solution I referred to earlier then? It’s an easy one fortunately: Make sure you check, double and triple check even, that the JDK your project is compiled with is exactly the same as the runtime you expect it to be executed in.
I’m super-duper grateful to you, @Graou74, for you taking the time to help me troubleshoot this. Muy appreciado!