Change Sidekiq testing mode to `fake` instead of `inline`

What does this MR do?

This changes the Sidekiq mode from inline to fake in the test environment.

This should hopefully make RSpec jobs run faster.

Documentation has been updated:

Sidekiq jobs are typically not run in specs, but this behaviour can be altered in each spec through the use of perform_enqueued_jobs blocks. Any spec that causes Sidekiq jobs to be pushed to Redis should use the :sidekiq_inline trait, to ensure that they are removed once the spec completes.

The :sidekiq_might_not_need_inline trait was added when Sidekiq inline mode was changed to fake mode to all the examples that needed Sidekiq to actually process jobs. Examples with this trait should be either fixed to not rely on Sidekiq processing jobs, or their :sidekiq_might_not_need_inline trait should be updated to :sidekiq_inline if the processing of background jobs is needed/expected.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• 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
• Security reports checked/validated by a reviewer from the AppSec team

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/16098.