Add a test to stab as not an operator

Author: josevalim

lib/elixir/test/elixir/code_fragment_test.exs

@@ -1050,6 +1050,9 @@ defmodule CodeFragmentTest do
  begin: {1, 5},
  end: {1, 6}
  }
+
+ # invalid
+ assert CF.surround_context("->", {1, 2}) == :none
  end
 
  test "sigil" do