[TIP] testing setuptools-related code

[Andrea Crotti]

I have (sadly) a special builder/runner that manages eggs and install eggs.
I've used the -m flag and I use pkg_resources to find the installed 
software,
but I didn't find a way to test everything..

So what might be the easiest/portable solution to write some tests that for
example:

- run the setuptools-based installer with the '-m'
- try to require it

without interfering with the global environment?