WIP: Adding tests to conda.install.rm_rf

This is to ready to merge and will not be ready to merge until all of the changes in `conda/install.py` have been undone.  As noted in 77f65c94579b6fe4d567d888958850ffb3b53e8c, the modifications there are to use this as TDD.  I started with a function that simply returned and have slowly been adding everything back in as each test is added to verify some additional functionality.

I'm pushing and opening the PR in case anyone's interested following along.
This function and its nesting is a good example of tests showing that there's too many dependencies here.  The problem here is the number of nested conditions inside this function.  There are ways to refactor it, but I didn't want to cloud this PR showing the refactor.  Instead, I wanted to focus on how to test parts of conda that are more complicated and show an example of hard-to-test code.