improve DoctrineDataCollectorTest and fix some deprecations

[dmaicher]

Fixes

Remaining self deprecation notices (1)

  1x: Method "Symfony\Bridge\Doctrine\DataCollector\DoctrineDataCollector::collect()" might add "void" as a native return type declaration in the future. Do the same in child class "Doctrine\Bundle\DoctrineBundle\DataCollector\DoctrineDataCollector" now to avoid errors or add an explicit @return annotation to suppress this message.
    1x in DoctrineDataCollectorTest::testCollectEntities from Doctrine\Bundle\DoctrineBundle\Tests\DataCollector

Remaining indirect deprecation notices (2)

  1x: Passing an instance of Doctrine\ORM\Mapping\ClassMetadataInfo to Doctrine\ORM\Tools\SchemaValidator::validateClass is deprecated, please pass a ClassMetadata instance instead. (SchemaValidator.php:82 called by DoctrineDataCollector.php:116, https://github.com/doctrine/orm/pull/249, package doctrine/orm)
    1x in DoctrineDataCollectorTest::testCollectEntities from Doctrine\Bundle\DoctrineBundle\Tests\DataCollector

  1x: DebugStack is deprecated. (DebugStack.php:41 called by Generator.php:715, https://github.com/doctrine/dbal/pull/4967, package doctrine/dbal)
    1x in DoctrineDataCollectorTest::testGetGroupedQueries from Doctrine\Bundle\DoctrineBundle\Tests\DataCollector