[#70513870] Unit tests and standardise testing section in README.

This is the only project that didn't have unit tests. Mostly because it's the
simplest - has no real logic of it's own, just uses Vcloud::Core. But it
would still benefit from having one. Mostly ripped off from Vcloud::Launcher
with the exceptions that:

- It calls describe on the object to be tested rather than namespacing it
  within a module as we plan to do for the other projects.
- Uses `let!` to separate variables from logic/expectations.
- It doesn't continue on errors like Vcloud::Launcher can.

It has the added benefit that:

- `rake spec` no longer executes all of the tests because the `Filelist[]`
  doesn't return any matches.
- We can make the testing section of the README look more like the other
  projects.

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README section copied from alphagov/vcloud-core@c8a2318