Set environment variable SYMFONY_DEPRECATIONS_HELPER for phpunit

[finnlewis]

Set environment variable SYMFONY_DEPRECATIONS_HELPER for phpunit to disabled to suppress display of deprecation notices in phpunit output.

[ekes]

@millnut

Hi @finnlewis I think we shouldn't change this as we'd miss deprecations that might not be picked up by automated tooling and can only be flagged at runtime, so it is good to have these reported so they can be looked at, logged and fixed. Looking at Drupal core they also don't have this set up.

localgovdrupal/localgov_project#157 (comment)