Before this commit, when we have a notification in the messaging menu (systray)
that comes from a message in inbox which isn't linked to a document
(e.g. a task), clicking on it result in a crash.
This issue comes from a typo of a recent js mail refactoring [1]
Although this is only a 1-line fix, there are no tests on the messaging menu
widget in the systray. Therefore, this commit also introduces some basic tests
of this widget, in addition to the test related to the fixed issue.
[1] https://github.com/odoo/odoo/commit/02ec09cb1c3e2d7bc7968f40c18f2208d7f3f498