Add test for mp_pop_back with value lists. Refs #53.

test/Jamfile

@@ -146,6 +146,7 @@ run mp_nth_element_q.cpp ;
 run mp_back.cpp ;
 run mp_back_2.cpp ;
 run mp_pop_back.cpp ;
+run mp_pop_back_2.cpp ;
 run mp_flatten.cpp ;
 run mp_rotate_left.cpp ;
 run mp_rotate_right.cpp ;