Fix doctrine unit test

PHP Fatal error:  Class JMS\Serializer\Tests\Fixtures\SimpleObjectProxy contains 6 abstract methods and must therefore be declared abstract or implement the remaining methods (Doctrine\Common\Proxy\Proxy::__setInitialized, Doctrine\Common\Proxy\Proxy::__setInitializer, Doctrine\Common\Proxy\Proxy::__getInitializer, ...)
Fix unit tests staticExpects() call

PHPUnit 3.9 removed support for staticExpects() on mock objects.
See https://github.com/sebastianbergmann/phpunit-documentation/issues/77
Fix unit tests using Symfony Validator >=2.2.0

[BC BREAK] inserted argument $message in the constructor of ConstraintViolation
https://github.com/symfony/symfony/blob/d57471ee63825d67c7372beb934d1dbe53283b73/src/Symfony/Component/Validator/CHANGELOG.md#220
Add tests for Doctrine discriminator inheritance
Use old array syntax
Fix unit tests using Symfony Validator >=2.2.0
Refactor doctrine drivers Scrutinizer feedback

https://scrutinizer-ci.com/g/schmittjoh/serializer/inspections/899f7307-649c-43c2-a399-1ede7d98b4e0/issues/files/src/JMS/Serializer/Metadata/Driver/DoctrineTypeDriver.php?status=existing&orderField=path&order=asc
Merge branches 'fix/discriminator-merging' and 'refactor/scrutinizer-doctrinedrivers'