fix(specs): stop sidekiq-scheduler after each test

[solnic]

Otherwise this may happen:

    1) Sentry::Sidekiq::Cron::Job sidekiq-cron adds job to sidekiq within transaction
      Failure/Error: expect(::Sidekiq::Queue.new.size).to eq(2)

        expected: 2
              got: 5

        (compared using ==)
      # ./spec/sentry/sidekiq/cron/job_spec.rb:124:in `block in <main>'

  Finished in 6.23 seconds (files took 4.01 seconds to load)
  75 examples, 1 failure, 2 pending

  Failed examples:

  rspec ./spec/sentry/sidekiq/cron/job_spec.rb:118 # Sentry::Sidekiq::Cron::Job sidekiq-cron adds job to sidekiq within transaction