Standardise testing section in README

Same as alphagov/vcloud-core@c8a2318f33b12828416c3a963ad0462ff4d8f906

I've omitted the fact that `integration` is aliased to `integration:all`
because I don't think most people need to know that. They can look in the
Rakefile if they're so inclined.

Retained an extra line about the quick tests though.
Merge pull request #27 from alphagov/70513870-readme_document_testing

[#70513870] Standardise testing section in README