Move misplaced alpha diversity tests

Unit tests for `single_file_cup` were in pycogent_backports tests instead of QIIME's alpha diversity tests. Also fixed issue where temporary files weren't being cleaned up.

Fixes #1488.
make biom strings pep8-compatible, fix cogent import

Thanks @josenavas!
Merge pull request #1508 from ElBrogrammer/cuptest

Move misplaced alpha diversity tests