test: add expectWarning to common

There are multiple tests that use the same boilerplate to test that
warnings are correctly emitted. This adds a new common function to do that
and changes the tests to use it.

PR-URL: https://github.com/nodejs/node/pull/8662


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>