Provide the ability for docker-based tests to execute on a full XS flavor

By default the docker-based tests build a minimal XWiki instance so that:

• it's faster to execute
• it allows us to catch missing dependencies
• we don't create unnecessary dependencies between modules/features

However, there are cases when we would like to run the tests on an instance being the same as the XS flavor distribution. For example, inside flavor docker tests, or if we want to take screenshots with a full XS UI, while being inside the docker tests of a specific module.