Update PHPUnit jobs to new driver when D11 "current" is W3C-compliant
Problem/Motivation
This is a follow-up to #3475581: Update Nightwatch jobs to new driver when D11 "current" uses Nightwatch 3 for making the similar changes to the PHPUnit jobs.
The changes here relate to the variable MINK_DRIVER_ARGS_WEBDRIVER
.test-variables has the definitions for the two variables MINK_DRIVER_ARGS_WEBDRIVER and MINK_DRIVER_ARGS_WEBDRIVER_LEGACY
Currently, in the committed repo, the phpunit 'current' job has an override to use the legacy value MINK_DRIVER_ARGS_WEBDRIVER: $MINK_DRIVER_ARGS_WEBDRIVER_LEGACY so all the variants would use this value, as they all extend from 'phpunit' (not '.phpunit-base').
However, the two 'next' variants contain
MINK_DRIVER_ARGS_WEBDRIVER:
!reference [.test-variables, variables, MINK_DRIVER_ARGS_WEBDRIVER]
meaning that they use the new value not the legacy one.
Proposed resolution
The new w3c driver should now be used for the 'current' and 'max PHP' variants as these run Drupal 11.1. Only the 'previous minor' 11.0 and 'previous major' 10.4 need to use the legacy value.
Specifying the values within the job variant is not entirely reliable, as the core version could be customised. It also leaves future work to be done when 'previous minor' becomes 11.1. So the proposal is to use the actual Drupal Core version being tested to determine whether to use the new or legacy value.
This will be done in the Composer job, creating a new derived variable INSTALLED_DRUPAL_VERSION which will be avilable for use in all subsequent jobs.
Remaining tasks
Related issue: Issue #3475581
Related issue: Issue #2829040