A big thank you Lingocoder, the eclipse block of the modulefy.gradle file exactly meets my expectations. This is precisely the behavior I wanted.
I wonder if I should not, once the Eclipse test run bug has been fixed, make a tutorial on this case.
I do not remember seeing this eclipse task script during my search on the web (or I should clean my glasses).
Anyway, I spent a lot of time on this problem (and you too) for finally something that I thought should have been plug & play.
I imagine that someone in my situation (new to Gradle and Java-9 modules) would be happy to find a ready-made solution.
Again thank you Lingocoder for your help, I will now be able to code in comfort.