[test] do not overwrite user's CW_CUBES_PATH in unittest_migration
Currently when you rely on your CW_CUBES_PATH environment variable to
locate cubes necessary for cubicweb's tests, the migration test crashes
because it overrides the env var.
Add some entities
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As manager, you can access to entity creation forms by using the `site management`_ page.
.. _`site management`: ../manage
As regular user, the index page or the action box may propose some links to create entities according to the context.