Refactor spec testing style

This should improve sidekiq handling across specs.

We will use tags to decide which sidekiq mode we want to use. This pull-request contains
`needs_hets` and `process_async` tags which will force sidekiq to run in `inline` mode, which
automatically process every job that would be put on the queue otherwise.

We should also use `needs_hets` everwhere where we need to interact with hets (as these tests are usually slow), as this will allow us to make a test run like this: `rspec --tag ~needs_hets` which will run all specs except those involving hets.