Add Gitlab Templates Downstream testing project

Migrated issue

Reported by: jonathan1055

Related to !324 (merged)

Problem/Motivation

To improve the Gitlab Templates MR test coverage we have created a project called Gitlab Templates Downstream which will be designed to cover several structural and configurational scenarios for D7 and D10 modules. See #3503337: Plan for initial codebase

So far, we have used KeyCDN, API and Decoupled Pages as convenient projects for downstream testing, given that one has to be a maintainer of the downstream project to be able to trigger the pipeline. These will remain as linked projects, but they do not cater for all the cases we need to test. The new project can be designed specifically to fill these gaps.

Proposed resolution

Add the pipelines for all the Gitlab Templates Downstream branches into this projects .gitlab-ci.yml so they can be run in MRs