Clear Sidekiq queues before each example

What does this MR do?

Recently there was an issue where a background migration spec had the :migration metadata but lacked the :sidekiq one.

I think it'd be safe to just clear the Sidekiq queues before each example to avoid these cases.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content