Draft: Generate a single predictive child pipeline

Context

Closes gitlab-org/quality/engineering-productivity/team#444

See gitlab-org/quality/engineering-productivity/team#427 (comment 1821337510).

In gitlab-org/quality/engineering-productivity/team#427, we wanted to run rspec:coverage and rspec:undercoverage in the backend predictive child pipeline, but we currently run two predictive child pipelines.

What does this MR do?

We used to create two separate child pipelines for EE and FOSS.

We now merge those two children pipelines together.

We could not "just merge them" ™️, because they have some CI/CD templating jobs that are identical. We first extracted what they had in common in another template, so that the final rendering was possible.

Proof of work

No tests to run

https://gitlab.com/gitlab-org/gitlab/-/pipelines/1221043587 ✅

Possible thanks to this line.

(fun fact: thanks to YAML, we generate three jobs named no-op because we have three empty files for the detected tests, and they are "merged together" into a single no-op job 🎉 )

Test is with only FOSS tests

https://gitlab.com/gitlab-org/gitlab/-/pipelines/1220975796 ✅

Possible thanks to this line.

Test is with only EE tests

https://gitlab.com/gitlab-org/gitlab/-/pipelines/1222271121 ✅

Possible thanks to this line and this line.

Test it with both FOSS/EE tests

https://gitlab.com/gitlab-org/gitlab/-/pipelines/1220908120 ✅

Possible thanks to this line

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