testcombo: Test grid :row- and :column-span-column

testcombo: Test grid :row- and :column-span-column

Ditch two items that were white and so weren’t visible on our standard
theme anyway, and use the new space to test extra grid-mode properties.

Note that if we do this then, as before, we set the ListStore on the
ComboBox before appending to it, that produced runtime warnings like:

Gtk-CRITICAL **: gtk_menu_attach: assertion 'left_attach < right_attach' failed

I didn’t look into that yet, but it may indicate that attaching items
vs. recognising their spans don’t occur in the correct order. For the
purposes of testing this, I just create the CB after filling its model.