Flush sidekiq queues before running import_rake_spec.rb

What does this MR do?

CI is failing due to import_rake_spec.rb intermittently failing. https://gitlab.com/gitlab-org/gitlab/-/jobs/366877840

This could be because of Sidekiq::Worker.drain_all that tries to execute leftover jobs from projects that were created in other specs and are no longer present.

This change makes sure Sidekiq queues are empty.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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