Added tests for GI_TYPE_TAG_UNICHAR

- forward_find_char() and backward_find_char() in GtkTextBuffer fail
  with Glib::Object::Introspection 0.015, because it doesn't know the
  GI_TYPE_TAG_UNICHAR type; added test case to show failure
Bump required version of Glib::Object::Introspection

- Newer version of G:O:I added handling for GI_TYPE_TAG_UNICHAR