tests: call g_type_init() if needed
tests: call g_type_init() if needed

On GLib older than 2.34.0, g_type_init() should be called before
gtk_init(). Let's hope the same goes for g_test_init().

This could possibly solve the problem on TravisCI that is still using
glib-2.32.4.

ntd/adg,ntd/adg