This is to reduce the load on CI server because of parallel clones of the
repository in Docker containers. Only keep "server", and "web" environments
separated. All other prior environments are in "misc".

To avoid duplicate entries in requirements files, move them all in a
"requirements" directory (this appears to be a "common" practice, see e.g.
Celery).

Adjustments in tests:

* Rename cubicweb/hooks/test/unittest_notification.py so that it does not
  conflict with cubicweb/sobjects/test/unittest_notification.py during test
  discovery as they would have the same module name but different __file__
  attribute.
* Add "comment" cube to the list of expected cubes in unittest_cwconfig.py as
  this cube is pulled by requirements/test-mist.txt.

Closes #15440662.