gl/frameworks: Use piglit_set_destroy_func() when gl_fw->destroy is set

Fixes: c2b31333 ("framework: call destroy directly instead of using atexit")
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Acked-by: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <!980>