mongodb: a simple test

This is a really simple test for MongoDB. It's a little more strenuous than `--version` or `--help`, but not by much. It outputs sysinfo. Right now, that's just the system page size.

I considered starting up a MongoDB instance, writing a document, reading it, and writing assertions. That's a much better test, but way too brittle. Plus it involves some teardown. I'm open to feedback on this if you guys would like to see a different approach pursued. Thanks!