Testing: Move ESLintTester to be external dependency (fixes #480)

This is my attempt to externalize ESLintTester. As I mentioned in the issue, there's a strange error I started getting when I did this. It came from the CLI unit tests. For some reason, running those tests plus a rules test resulted in the default config not working properly. I tracked the issue down to optimist, then minimist, where it seems to be including the same aliased value twice, making `config.format` into an array with two items rather than a string value. However `config.f` is always just a single string value, so I switched to using that.

The config.js file is a bit of a mess, I think it's in need of cleanup and we may eventually figure out what the issue is at that time. In the meantime, this fix works, even if I'm not sure what the real problem is.

This is mostly just a proof on concept for @ilyavolodin to look at. If it all seems okay, I can work on getting it merge-ready.