Write tests for rez-pip

Now that we are done with `rez-pip`, we need tests to make sure it works and that it doesn't break in the future (with new versions of pip, python, etc).

The most important tests to write for now are smoke tests. That is tests that ensure we can install a simple package (so that we can go through the entire process, though not all branches in the code).

Then we could add more tests to test all branches of our code.

Requirements (from my point of view):
* We should not download packages from PyPI. We can use https://github.com/pypa/pipenv/blob/master/tests/pytest-pypi/setup.py to emulate a local pypi with pytest. THis local pypi would use local packages.
* Have short and long tests. We could easily have tests that run everytime and tests that run only when requested (like installing the top 100 pypi packages straight from pypi for example).

Let me know what you think on this.

Start by locating the rez-pip implementation and existing test setup in the repository, then review the linked pipenv pytest-pypi setup.py example for a local package index. Done means adding smoke tests that install a simple local package without downloading from PyPI, with short and optionally requested long test coverage.

Written by the indexing model from the issue text.