Unit tests for Terminus helper functions

The following provides test coverage for the following helper functions found in terminus.drush.inc:
* terminus_validate_environment
* terminus_is_reserved_branch_name
* terminus_validate_new_branch_name
* terminus_validate_uuid

References #44

Day 3 of my [24 Days of Pull Request](http://24pullrequests.com/users/desmondmorris)