Collect test deps in requirements file

In #1488 we missed installing the 'py' library before installing
py.test, so pip was downloading that from the network even though we had
it in vendor already. We also didn't remove the nose tarball from
vendor. This addresses the latter and addresses the former by factoring
out a requirements.test.txt file and pointing to that from Makefile.
Rename to requirements_tests.txt

This is parallel to default_tests.env.
Merge pull request #1505 from gittip/test-requirements

Install test deps from a requirements file