Add Sidekiq middleware stack integration tests

What does this MR do?

Before this change, Sidekiq middleware components were only tested through unit tests. No integration testing of Sidekiq middleware components was done.

This state of affairs led to this change passing all tests and being merged into master: !19491 (merged)

Due to a type error, the change did not work, but without integration tests, this was only discovered when it was deployed to dev.gitlab.org, where it broke all background processing jobs.

This change adds basic integration tests for our server-side and client-side middleware stacks, to ensure that messages pass through the stack without causing exceptions.

The change also ensures that each middleware yields control as expected.

cc @DylanGriffith @engwan

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