A few fixes to qiime_test_data stuff

This pull request includes a few different fixes. Sorry for mashing them into one PR, but they were all relatively small and related to the recent ```qiime_test_data``` move.

Changes:

- Removed ```qiime_test_data_dir``` from print_qiime_config.py check
- Updated all_tests.py to have a better error message for when the ```qiime_test_data``` directory cannot be found (shouldn't happen, but still good to have the check)
- Changed all_tests.py ```--script_tests``` to ```--script_usage_tests``` to be consistent with our naming scheme and ```--suppress_script_usage_tests```
- Removed reference to deprecated ```qiime_test_data``` repository
- script usage tests are now sorted before being run (wasn't an issue on Mac OS X, but was for Ubuntu)

Fixes #852, #853, and #854.