require.undef()

Provides require.undef() support in node and browser tests

I finally had some time to figure out the missing pieces.  It turns out that Intern has a dependency (in util.js) on a few node modules defined internally in its own package.json.  Those missing packages (e.g. istanbul) weren't being found by dojo/node! because they were not in a node_modules folder that was an ancestor to dojo.

I'm not sure if including the modules in our package.json is the best way to go about this.  There could be a better way to go about remapping dojo/node! to load from Intern's node_modules while inside intern.

Colin suggests that we can fix the system to not load those node modules through realClient.

I figured that at the very least this PR can open the conversation.