Ensure $HOME is set to `node[:rbenv][:user]` before git clones

This is a workaround for CHEF-3940. TL;DR Certain versions of
`git` misbehave if configuration is inaccessible in $HOME.

We work around this issue by ensuring $HOME is temporarily set to the
`node[:rbenv][:user]` user before performing a `git` operation.
Add Test Kitchen support and some initial Bats integration tests.