Add @testset to for loops in tests (#420)

* add `@testset` to for loops

* add more loops

Author: jishnub

test/Poly.jl

@@ -387,7 +387,7 @@ p2s = Poly([1], :s)
 
 # test iteration
 p1 = Poly([1,2,0,3])
-for term in p1
+@testset for term in p1
   @test isa(term, Poly)
 end