I would format these tests a bit differently:
```
format_test_() ->
    [
     {"formatter returns empty list when value is undefined",
      ?_assertEqual([], oc_wm_request_logger:format_note(key, undefined))},

     {"formatter returns empty list when value is empty string",
      ?_assertEqual([], oc_wm_request_logger:format_note(key, ""))},

     {"formatter removes empty and undefined values from proplist",
      fun() ->
              PropList = [{key1, undefined},
                          {key2, ""},
                          {key3, "value3"}],
              Got = iolist_to_binary(oc_wm_request_logger:format_note(proplist, PropList)),
              ?assertEqual(<<"key3=value3; ">>, Got)
      end}
    ].

```