is Rails.cache purged between tests?

A more efficient (and easier) method is to set the test environment’s cache to use NullStore:

# config/environments/test.rb:
config.cache_store = :null_store

The NullStore ensures that nothing will ever be cached.

For instance in the code below, it will always fall through to the block and return the current time:

Rails.cache.fetch('time') { Time.now }

Also see the Rails Caching guide: http://guides.rubyonrails.org/caching_with_rails.html#activesupport-cache-nullstore