Disallow usage of Sidekiq::Testing.inline! without a block in specs

[Bennet-Sunder]

From Sidekiq's testing guide, it is suggested to use Sidekiq::Testing.inline! with a block so that it doesn't share state.

# or use block mode to avoid leaking the setting into other tests
Sidekiq::Testing.inline! do
  HardWorker.perform_async
  assert_worked_hard
end

https://github.com/sidekiq/sidekiq/wiki/Testing#testing-workers-inline

So this should throw a warning for the below spec

Sidekiq::Testing.inline!
HardWorker.perform_async
assert_worked_hard

[pirj]

We don’t keep cops that relate to testing third-party libraries in rubocop-rspec. See #1374
#1646
We’ve even extracted Capybara and FactoryBot, and plan on extracting rubocop-rspec_rails.