Allow Traits to define modules for install during tests

Is there an example test that needs that could be converted.

This might be better defined as a feature request.

Let’s see what committers think!

I'm not sure about this one. Drupal is really special in how we futz with the autoloader to make test traits autoloadable (and in fact all the tests). This is not normal behaviour.

The issue summary needs to be updated with a concrete use case of why this is or might be a good idea. Atm the issue summary is just about why you can't do it not why to do it.