Cleanups for import unittest/unittest2

This change was made mechanically using sed, except for some manual edits in
test_build_graph.py and test_thrift_util.py

(I realize this change will be pretty much a no-op after we drop python 2.6 support)

Testing Done:
CI is running @ https://travis-ci.org/pantsbuild/pants/builds/32971877

Bugs closed: 500

Reviewed at https://rbcommons.com/s/twitter/r/919/
Put back error message that shows path to missing BUILD files.
Regressed from  https://rbcommons.com/s/twitter/r/841/