Set the correct target_core in all phpunit variants

Migrated issue

Reported by: jonathan1055

Related to !103 (merged)

Problem/Motivation

When testing using OPT_IN_TEST_PREVIOUS_MAJOR the composer step redefines the variable _TARGET_CORE: $CORE_PREVIOUS_STABLE which is good, it works fine. The phpunit job confirms via drush status that it is running Drupal version : 9.5.11

However, if you do a conditional test in after_script based on the variable _TARGET_CORE is does not retain 9.5.11 and has the standard value which is currenty 10.2.x-dev

Cause

composer (previous major): has

  variables:
    _TARGET_PHP: $CORE_PREVIOUS_PHP_MIN
    _TARGET_CORE: $CORE_PREVIOUS_STABLE

But phpunit (previous major) only has

  variables:
    _TARGET_PHP: $CORE_PREVIOUS_PHP_MIN

(thanks to fjgarlin for this hint, when I asked on Slack)

Proposed resolution

Add the missing variables to all the phpunt variants