CI::Queue::Static and CI::Queue::File initialization is broken

After https://github.com/Shopify/ci-queue/pull/34, you need to initialize the File and Static queue passing the file name or the test respectively and you also need to populate the queue with the list of tests. This seems contra-intuitive.

Maybe the initialize should only take the config and the populate the list of tests and add a .populate class method (or any other name) that does what the initialize used to do.

Also those queues require a configuration object that is not needed in most of its use cases. Making it optional, or adding a high level method as .populate that creates the configuration object would be great.

cc @casperisfine @wvanbergen @tjoyal

Nitpick:

The @index instance variable is only set after you call #populate so if you don't you get a "non-method error fetch for nil" in the to_a method and a ruby warning because the instance variable is not initialized. Maybe we should try to write warning free code so we could avoid this kind of error?

Start by tracing initialization for CI::Queue::Static and CI::Queue::File, then inspect how #populate and #to_a interact with the queue list and @index. Clarify the intended API for configuration, test population, and the proposed .populate class method before changing it. Done means common use cases need no unnecessary configuration and unpopulated queues do not produce nil-method errors or Ruby warnings.