Move the tests out of the main app directory

This project layout worked really well with Plata and Towel.
Make the RSSContent test run faster by providing a local RSS file

Refs #313.
Small test speedup (2-3sec) by not going through the admin all the time

Refs #313.
Fix sitemap generation when there are no pages yet

Thanks to javisto for the report.

Fixes #393.
Merge branch 'master' into next

* master:
  Fix sitemap generation when there are no pages yet