Use os.path.normpath before os.path.basename

Testing Done:
[tw-mbp13-tdesai pants]$ ./pants tests/python/pants_test/base:address
Loading pants backends from source
Build operating on top level addresses: set([BuildFileAddress(tests/python/pants_test/base/BUILD, address)])
================================================================================ test session starts ================================================================================
platform darwin -- Python 2.6.7 -- py-1.4.20 -- pytest-2.5.2
plugins: cov, timeout
collected 3 items

tests/python/pants_test/base/test_address.py ...

============================================================================= 3 passed in 0.06 seconds ==============================================================================
tests.python.pants_test.base.address                                            .....   SUCCESS
[tw-mbp13-tdesai pants]$

Bugs closed: 209

Reviewed at https://rbcommons.com/s/twitter/r/499/