Add 'panels_test' module that we can use for functional tests

At the moment, it's impossible to write functional tests for the forms provided by the PanelsDisplayVariant because there's no way to use the variant without another module, like page_manager.

This issue is to add a hidden 'panels_test' module which depends on page_manager!

It doesn't include any actual tests (there are some in #2249401: Unable to save layout changes on panel page) but we need to merge this one first so that the testbot can scan the repo and know that it has to download the page_manager module when running the Panels tests.