Move Pipeline#merge_request_event_type => :merge_train into EE

What does this MR do?

This MR adds specs that had surfaced a bug in Ci::Pipeline#merge_request_event_type that's since been corrected in another MR. I think it's worth contributing these specs and the additional Pipeline factory, as they will do a better job of specifying the behavior and make the factory available for writing other specs around merge train pipelines.

In review, we also decided to move the logic for determining that a Pipeline's MR event type is :merge_train into EE, since Merge Trains are an EE feature and generally not used or referenced in the FOSS version of Gitlab.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] 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