Rename mympz.h --> test-obj.h

as it is used only for testing purpose.

tests/test-msnapshot.c

@@ -20,7 +20,7 @@
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
-#include "mympz.h"
+#include "test-obj.h"
 
 #include "m-snapshot.h"
 #include "m-mutex.h"