Test that the on-screen keyboard works and its layout is correctly set

The screen keyboard must:

work in Tor Browser when activated after the browser has started;
work in Thunderbird when activated after Thunderbird has started;
be auto-configured to use the same keyboard layout as the X session (this does not work yet with GNOME’s on-screen keyboard, see #8444 for details).

Parent Task: #10250

Original created by @kytv on 10263 (Redmine)

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