internal/fields: fix test to match next Go version

In Go 1.10, encoding/json will no longer allocate pointers to
embedded unexported structs. Fix a test to allocate the struct
for it. We were already doing this for the fields package, so it
seems fair.

Change-Id: I3118526025a128f26f9308d6904d500d89ad01e3
Reviewed-on: https://code-review.googlesource.com/20672
Reviewed-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Joe Tsai <joetsai@google.com>
Reviewed-by: Michael Darakananda <pongad@google.com>
diff --git a/internal/fields/fields_test.go b/internal/fields/fields_test.go
index 136eabe..925765d 100644
--- a/internal/fields/fields_test.go
+++ b/internal/fields/fields_test.go
@@ -142,9 +142,10 @@
 		Anonymous: Anonymous(15),
 	}
 	var want S1
+	want.embed2 = &embed2{} // need this because reflection won't create it
 	jsonRoundTrip(t, s1, &want)
 	var got S1
-	got.embed2 = &embed2{} // need this because reflection won't create it
+	got.embed2 = &embed2{}
 	fields, err := NewCache(nil, nil, nil).Fields(reflect.TypeOf(got))
 	if err != nil {
 		t.Fatal(err)