Use Tempfile as cached image in tests

This is a quick follow-up to https://github.com/chrishunt/desktop/pull/20.

We are now passing the cached image path through so we can use a `Tempfile` in the tests. After we merge this PR, we'll no longer be deleting the cached desktop image from the host machine running the tests :smile: 

There are also a few other small things in here:
  - rename `cached_desktop_png` to `cached_image`
  - remove `sudo` and `-rf` from `remove_cached_image`
  - silently fail if cached image cannot be found (no need to check first)