Add __lager_test_sink to rebar.config

rebar.config

@@ -1,4 +1,4 @@
-{erl_opts, [debug_info, warn_untyped_record]}.
+{erl_opts, [{lager_extra_sinks, ['__lager_test_sink']}, debug_info, warn_untyped_record]}.
 {erl_first_files, ["src/lager_util.erl"]}.
 {deps, [
         {goldrush, "0\.1\.6",