Clean up test output

The test runner currently has a lot of noise that we don't need to see when running the tests. Let's clean it up :sparkles:

Most of the output in the tests is from overzealous logging that doesn't need to happen, so that's been quieted down.

The other main culprit is UTF-8 encoding warnings in the `MailSender`. Those warnings were fixed by [explicitly setting the charset](https://github.com/chrishunt/discourse/commit/e94e9b7621215a9493b099686b102bdd41425fe6) of plain text emails to UTF-8, just like we [already do](https://github.com/discourse/discourse/blob/4cf1d9c2667b80e8cc18b2929b14a6b938cc16e3/lib/email_sender.rb#L26) for the html part.

This PR changes this:

![](https://dl.dropboxusercontent.com/u/856934/ugly-tests.jpg)

To this:

![](https://dl.dropboxusercontent.com/u/856934/nice-tests.jpg)