Purge not resetting Ids when using LoadFixtures with Doctrine

I expect to work with certain Ids within my tests but doing

```
$fixturePath = "@Bundle/DataFixtures/ORM/Test/yml/";

$fixtures = array(
$fixturePath . 'AuthUser.yml',
);

$this->loadFixtureFiles($fixtures);
```

Does not resets the ID on AuthUser, causing it to grow exponentially with every test wich not ideal, with SQLlite there's an implicit datasedrop to solve that

Start by tracing loadFixtureFiles with Doctrine-backed tests and inspect how purge state is handled between fixture loads. Compare the behavior with SQLite's database reset behavior; done means test fixture IDs reset consistently between tests without breaking the existing SQLite case.

Written by the indexing model from the issue text.