ERP5TypeTestCase: rework default manager password generation

The default manager user (ERP5TypeTestCase) now have a random password
generated at the beginning of test. The password is stored as a class
attribute of the test case instance.
Many tests have been updated to not generate manager user and use the
existing one instead. When tests need to create users (for example
users in the root acl_users), we try to give them a random password and
to delete the users afterward.

For functional tests, the approach is that ERP5TypeFunctionalTestCase
sets cookies with manager username and password before running zelenium
tests, so that in case tests want to log in again as manager, they can
read the username and password from cookies.

Another significant changes is that we no longer have the same user in
ERP5/acl_users and /acl_users, some tests were logging in as the root
user in ways that never seemed intentional.

This also revealed (through test_manager_actions_on_portal from
testERP5Core) that some tests were running with a user without all the
expected permissions and the "Manager Components" actions was not
visible for the default manager user. Fixing this also revealed that
two actions ("Manage Components" and "Manage Callables") had the same
priority, so the later was modified to use a different priority.