Check the uniqueness of the values of pipeline enum

What does this MR do?

In the https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23217, we added a new key & value pair to Ci::PipelineEnums. However, the value was actually conflicted with one of the EE specific pairs. Luckily, there was one spec (only one!) failed because of the conflict, however, if we didn't notice, we'd have to go down a tough path to correct the data integrity by shipping another database migration.

We should check the uniqueness of the enum values.

What are the relevant issue numbers?

• Related https://gitlab.com/gitlab-org/gitlab-ce/issues/15310
• Related https://gitlab.com/gitlab-org/gitlab-ee/issues/8451
• Related https://gitlab.com/gitlab-org/gitlab-ee/issues/8449

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer