mxnet integration test

test/backend.jl

@@ -1,15 +1,24 @@
 xs = rand(20)
 d = Affine(20, 10)
 
-@tfonly let dt = tf(d)
-  @test d(xs) ≈ dt(xs)
-end
+# MXNet
 
 @mxonly let dm = mxnet(d, (1, 20))
   @test d(xs) ≈ dm(xs)
 end
 
-# TensorFlow native integration
+@mxonly let
+  using MXNet
+  f = mx.FeedForward(Chain(d, softmax))
+  @test isa(f, mx.FeedForward)
+  # TODO: test run
+end
+
+# TensorFlow
+
+@tfonly let dt = tf(d)
+  @test d(xs) ≈ dt(xs)
+end
 
 @tfonly let
   using TensorFlow