Refactor CI processable delegation specs

What does this MR do?

This MR is a follow-up from !27968 (comment 323086163) to more simply test the delegation of methods from Ci::Processable to Ci::Pipeline. Repeating the outcome requires a bunch of setup for no real additional value over the specs that exist for the method on Ci::Pipeline.

Merging !27968 (merged) is a requirement for this, and this MR will be rebased on master at that time. After that this MR will consist of a single commit: !29605 (ed826ae2)

https://about.gitlab.com/handbook/values/#cleanup-over-sign-off

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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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