Nightwatch testing against all opted in versions

Issue information

From user: wim leers

Related to !78 (merged)

Contribution record

Fork management

Problem/Motivation

1. #3396106: Allow modules to opt in to testing against Drupal 11 even before they support it already landed and added composer (next major) + phpunit (next major).
2. Once #3375359: Nightwatch tests do not work with the default template: port the approach from core is fixed, Nightwatch tests will work out of the box for the current minor.
3. Once #3397129: Allow modules to opt in to testing against Drupal previous major, previous minor, next minor is fixed, Nightwatch tests could work for all opted in core versions.

Steps to reproduce

Proposed resolution

1. Wait for #3375359: Nightwatch tests do not work with the default template: port the approach from core to land, then add nightwatch (next major) job.
2. Wait for #3397129: Allow modules to opt in to testing against Drupal previous major, previous minor, next minor to land, then add nightwatch (previous major), nightwatch (previous minor) and nightwatch (next minor) jobs.

Acceptance criteria: all customzied and newly added jobs in https://git.drupalcode.org/project/decoupled_pages/-/blob/8.x-1.x/.gitlab-ci.yml disappear and are replaced with a number of OPT_IN_* variables!

Remaining tasks

User interface changes

API changes

Data model changes