testattributes: Make it work with --enable-debug=no
testattributes: Make it work with --enable-debug=no

g_assert gets neutered by --enable-debug=no, which is not what
we want in tests. Use g_assert_true instead.

https://bugzilla.gnome.org/show_bug.cgi?id=755126

Distrotech/pango,Distrotech/pango,Distrotech/pango