On some slower machines (e.g. an ARM OBS builder), this test is failing
with a race condition where we're trying to fetch the style before it's
applied.
https://bugzilla.gnome.org/show_bug.cgi?id=749593
gtk_tree_selection_real_modify_range() has a g_return_if_fail() if the
start or end paths passed to it do not correspond to real tree nodes.
However, GtkTreePaths inherently do not have to be valid, so it should
be acceptable to call gtk_tree_selection_select_range() with
non-existent paths. Replace the g_return_if_fail() by a silent return,
and add a unit test.
https://bugzilla.gnome.org/show_bug.cgi?id=712760
