GNOME on-screen keyboard is broken without the X11 XTEST extensions

Originally created by @anonym on #14675 (Redmine)

    Sep 16 08:05:27 amnesia caribou[10756]: g_object_unref: assertion 'G_IS_OBJECT (object)' failed
    Sep 16 08:07:57 amnesia org.gnome.Shell.desktop[9892]: Xlib:  extension "XTEST" missing on display ":1".

Can we make #14623 and #8281 (closed) to play nice with each other?

Feature Branch: bugfix/14675-re-enable-X11-testing-extension

Related issues

• Related to #10263 (closed)
• Related to #14623
• Related to #8281 (closed)
• Related to #12213