Drupal CMS variant

Migrated issue

Reported by: fjgarlin

Related to !425 (merged) !418 (merged)

Problem/Motivation

Talked about very briefly at DrupalCon Vienna with @phenaproxima.

Drupal CMS is a key project now, so we should check if modules are compatible with it by running CI on a Drupal CMS installation.

Proposed resolution

Allow modules to test whether they would be compatible with Drupal CMS by turning on a switch, like other variants.

Remaining tasks

• Are we testing just one version? Test latest stable.
• Do we need to run any additional tests other than running the normal CI flow of the module with Drupal CMS instead of just core? Yes, for recipes. See first comment.

User interface changes

We will see a new variant in the pipelines.

API changes

We will have a new variable OPT_IN_TEST_DRUPAL_CMS to enable the 'Drupal CMS' pipeline job. It will be off by default. See the documentation page for more details

Related issue: Issue #3552331

Related issue: Issue #3546508