mouse-panel: Add "Test Settings" to mouse-panel

This relates to issue #2839 (closed) when resizing the Mouse and Touchpad panel window to its minimum adaptive size, the title looks very ellipsized.

Initial implementation to move the "Test Settings" button from the header to the AdwPreferencesPage of the mouse-panel.

First commit removes the Test Settings button from the header of the mouse-panel.

Second commit adds Test_Settings as a child element of GtkOverlay into the mouse-panel for both the mouse and touchpad windows.

@felipeborges Let me know if any changes are to be made.

Screenshots repo attached: https://gitlab.gnome.org/Atbrat/changes_screengrabs/-/tree/main/gnome-control-center-grabs

Changes in action:

Screenrecord_control_center.mp4