Fixes https://github.com/pyugrid/pyugrid/issues/24 and passes PEP8.

Adds a context manager to switch tests into the proper directory for loading test files. Bonus: fixed some spacing that had used tabs and PEP8-afied test_read.py.
We may want to move this to a test utilities module at some point, but as this is th only file that needs it now, it's fine here.