Add mink test

This just tests a simple issuing of a tax receipt and is an alternate to !109 (closed). The main part of this MR though is the base class, which provides some helpers but more importantly gets around a chicken and egg problem. The mink tests are integrated with drupal from the drupal side, meaning your tests are expected to be in a drupal module. The temporary simpletest subfolder where the tests run doesn't exist yet before the test starts, and civi isn't installed yet, so there's no way for you to tell civi ahead of time where your extension code is, and by default it looks in that subfolder. This allows specifying a variable in phpunit.mink.xml to tell it where your extensions dir is.

Copy phpunit.mink.xml.dist to phpunit.mink.xml and edit. The rest of the instructions are currently pending at documentation/docs/dev!880 (merged)