func MustJSONMarshall(t *testing.T, a interface{}) []byte {
  res, err := json.Marshal(a)
  if err != nil {
    t.FatalF("err: %s", err)
  }
  return res
}
func TestWhatver(t *testing.T) {
  elem := MyStruct{Field:"something"}
  elemB := MustJSONMarshall(t, elem)
  ... 
}