Do not trigger a deprecation warning on SYMFONY_DEPRECATIONS_HELPER when DRUPAL_DEPRECATION_FILTER_CONFIG is also present

Problem/Motivation

In #3589108: Refactor DeprecationHandler into a standard PHPUnit extension, a deprecation warning was added in \Drupal\Tests\DrupalTestCaseTrait::checkLegacySymfonyDeprecationHelperEnvVariable() if the SYMFONY_DEPRECATIONS_HELPER environment variable is present.
This warning is even triggered if the new DRUPAL_DEPRECATION_FILTER_CONFIG variable, the new way of configuring the deprecation helper, is present. This makes it harder for contrib module to support both Drupal 11.5+ and older Drupal versions with the same code. (They either need to ignore the deprecation warning or set the env vars conditionally.)

Steps to reproduce

In a contrib module, define a deprecation ignore file in both the old and the new way.

Proposed resolution

My proposal would be to not emit the warning if the DRUPAL_DEPRECATION_FILTER_CONFIG env var is present. This would make it much easier for contrib modules to be compatible with both.

It is also how we handled other deprecations in the past: For instance, if a plugin uses both annotations and attributes, no deprecation warning is triggered.

Remaining tasks

• Decide whether this is intended behavior or whether it should be changed.
• In the latter case, merge the MR.

User interface changes

None.

Introduced terminology

None.

API changes

None.

Data model changes

None.

Release notes snippet

N/A