Add a test for mp_back with value lists. Refs #53.

test/Jamfile

@@ -143,6 +143,7 @@ run mp_max_element_q.cpp ;
 run mp_nth_element.cpp ;
 run mp_nth_element_q.cpp ;
 run mp_back.cpp ;
+run mp_back_2.cpp ;
 run mp_pop_back.cpp ;
 run mp_flatten.cpp ;
 run mp_rotate_left.cpp ;