events,test: fix TypeError in EventEmitter warning

Allows Symbol to be converted to String so it can be included in the
error.

Fixes: https://github.com/nodejs/node/issues/9003
PR-URL: https://github.com/nodejs/node/pull/9021


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>