Issue: wysiwyg asset slow down the loading of the website, error
inadvertently introduced: https://github.com/odoo/odoo/pull/29775
The assets are now loaded assynchroneously, when the editor is needed,
its assets will be loaded.
closesodoo/odoo#30700
The refactoring also had to be done in order to create a set of unit
tests.
Each behavior can be tested, including the behaviors performed as a
consequence of keyboard interactions (for instance: Enter, Tab...).
This commit also contains some changes to test_utils that were
necessary given the new structure of the wysiwyg editor.
Co-authored-by: Gorash <chm@odoo.com>