Add more UI tests for rustdoc

[GuillaumeGomez]

It is linked to #60033.

This is the tests we need to add (the list is non-exhaustive):

• a UI test for each setting
• click on element in the sidebar hidden by default to be sure it gets fully un-collapsed and scrolled to
• make tests for each shortcut
• make tests for source code navigation (through files in the first time should be enough)
• run all the previous tests in mobile mode