Introduce PHPUNIT_FAIL_ON_PHPUNIT_DEPRECATION
Problem/Motivation
PHPUnit 11 separated reporting/failing on PHP deprecations vs on PHPUnit internal ones.
PHP deprecations use the @trigger_error('...', E_USER_DEPRECATED); mechanism; PHPUnit internal use PHPUnit's event system. The two streams are reported separately in PHPUnit CLI with two specific arguments, --fail-on-deprecation and --fail-on-phpunit-deprecation.
run-tests.sh by default sets both options; in core, the PHPUNIT_FAIL_ON_PHPUNIT_DEPRECATION environment variable can be used to disable PHPUnit deprecations while keeping the PHP ones failing the tests.
See https://git.drupalcode.org/project/drupal/-/blob/11.x/.gitlab-ci.yml?ref_type=heads#L70
Proposed resolution
Introduce the same variable in the template.
Default value to be discussed.