Added test for user op in MPI.Reduce

Author: tkonolige

test/test_reduce.jl

@@ -18,6 +18,9 @@ val = rank == root ? size-1 : nothing
 val = rank == root ? 0 : nothing
 @test MPI.Reduce(rank, MPI.MIN, root, comm) == val
 
+val = rank == root ? size : nothing
+@test MPI.Reduce(1, +, root, comm) == val
+
 mesg = collect(1.0:5.0)
 sum_mesg = MPI.Reduce(mesg, MPI.SUM, root, comm)
 sum_mesg = rank == root ? sum_mesg : size*mesg