break breaks push/pop_testset invariance

[tkf]

julia> using Test

julia> @testset for i in 1:1
           @testset begin
               break
           end
       end
1-element Array{Any,1}:
 Test.DefaultTestSet("i = 1", Any[Test.DefaultTestSet(#= circular reference @-2 =#)], 0, false)

julia> Test.get_testset_depth()  # != 0
1

julia> @testset for i in 1:1  # same testset again
           @testset begin
               break
           end
       end
1-element Array{Any,1}:
 Test.DefaultTestSet("i = 1", Any[Test.DefaultTestSet(#= circular reference @-2 =#)], 0, false)

julia> Test.get_testset_depth()  # != 0
2

I think the code above should be an error. Maybe @testset should be lowered into an "immediately invoked function expression"?