ListViewTestModule: revert change accidentally added in e4412af0

This commit from Bertrand [1] included an accidental change to
ListViewTestModule that made the ListView shown in "make hg"
have less rows, which is not good for scrollbars showcase.

[1] https://git.gnome.org/browse/hyena/commit/?h=gtk3&id=e4412af083b0e11d993bb4660650a9e7485769c6