Add functional tests for grabEntityManager(), resetDoctrineManager() and grabContainer()

[TavoNiievez]

Covers the three accessors added in Codeception/module-symfony#248.

• grabEntityManager() — asserts it returns an open EntityManagerInterface, and that it is the very instance the application uses rather than a copy, by comparing it against doctrine.orm.default_entity_manager.
• resetDoctrineManager() — asserts both halves of its contract: an open manager is only cleared, so a previously managed entity leaves the identity map; a closed one is reopened. The trailing seeNumRecords() proves the transaction the Doctrine module opened for the test survived the reset.
• grabContainer() — asserts it returns the test container rather than the kernel's own. The last two assertions are the point of the method: the test container resolves a private service that the kernel container cannot see, which is what made the widespread grabService('kernel')->getContainer() workaround subtly wrong.

No app fixtures are needed. The reset test closes the manager itself instead of provoking a failed flush, which keeps it deterministic under settings.shuffle.

The four branch PRs are byte-identical apart from the Security import that already differed per branch. The module recovers through Doctrine's registry where the entity manager service is lazy and by rebooting the kernel where it is not — Symfony 5.4 — but both paths are observable only as a reopened manager, so no version-gated variant is needed.

These tests pass once composer.lock points at a module-symfony revision containing Codeception/module-symfony#248. Verified locally against that branch on Symfony 5.4 and 8.1: the full Functional suite is green.