Allow modules to opt in to testing against Drupal 11 even before they support it

Migrated issue

Reported by: wim leers

Related to !66 (merged)

Problem/Motivation

I was inspired by the DrupalCon Initiatives keynote, where @fjgarlin demonstrated that the Drupal core GitLab CI pipeline can automatically generate a test-only patch.

That got me thinking: surely it must be possible to automatically test contrib modules against Drupal 11 too then? 🤓

So I tried and … it turns out it's surprisingly simple: #3396093: Use GitLab CI to test Drupal 11 support.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Related issue: Issue #3396093

Related issue: Issue #3387322