tests: Fix bug in pixdata test

In 3724a739 we added a test for the deserializing code, but we shouldn't
be free'ing the contents straight away, as gdk_pixdata_deserialize()
does not copy the data, but expects it to stay around until we've copied
the data to the pixbuf (with gdk_pixbuf_from_pixdata() and copy_pixels
== TRUE), or we're done with the pixbuf.

https://bugzilla.gnome.org/show_bug.cgi?id=775693