[FIX] web: prevent some tests from failing undeterministically

This is a tricky one. The problematic tests involve the quick edit
feature. Before this commit, those tests sometimes failed [1],
because there was no guarantee that we waited enough in the test
before doing assertions.

[1] https://runbot.odoo.com/web#id=2012&action=573&model=runbot.build.error&view_type=form&cids=1&menu_id=405

Part-of: odoo/odoo#78621