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 is the only change that's really required.  Having this private function makes it hard to actually determine that it was called correctly and doesn't allow for any testing of that function to ensure it behaves the way it should.  I've extracted it into a tested function by itself.