Fix setuptools tests for macos
The setuptools tests in cffi0/test_zintegration.py were failing to install the generated packages on MacOS when the build/install was occurring at the same time due to a weird and obscure bug in setuptools' copy of easy_install. The tests would succeed on subsequent runs, since they don't clean up the builds in between. Switched the tests to use pip from the virtualenv to install, which isolates the builds and makes it work everywhere.