为什么我的测试不能在MySQL上运行？

我正在研究一个Symfony 2项目，该项目针对测试数据库运行PHPUnit 4.6.2进行测试。该测试数据库在配置文件中设置。以下是有关教义配置的部分：

config.yml：

doctrine:
    dbal:
        default_connection:       default
        connections:
            default:
                driver:   "%database_driver%"
                host:     "%database_host%"
                port:     "%database_port%"
                dbname:   "%database_name%"
                user:     "%database_user%"
                password: "%database_password%"
                charset:  UTF8

    orm:
        auto_generate_proxy_classes: "%kernel.debug%"
        auto_mapping: true

config_dev.yml：

imports:
    - { resource: config.yml }

config_test.yml：

imports:
    - { resource: config_dev.yml }

doctrine:
    dbal:
        default_connection:     test_sqlite
        connections:
            test_sqlite:
                driver:   pdo_sqlite
                path:     %kernel.cache_dir%/test.db
            test_mysql:
                driver: pdo_mysql
                host: 192.168.56.2
                port: null
                name: myproject_test
                user: root
                password: mysupersafetestpassword
                charset: UTF8

只要我保持config_test.yml中test_sqlite的值doctrine.dbal.default_connection，测试就可以正常运行。但是，当我将此值更改为test_mysql我的每个测试时，都会收到以下错误消息：

无法将别名“ doctrine.dbal.test_mysql_connection”替换为“ database_connection”。
 C：\ projects \ myproject \ vendor \ symfony \ symfony \ src \ Symfony \ Component \ DependencyInjection \ Compiler \ ReplaceAliasByActualDefinitionPass.php：48
 C：\ projects \ myproject \ vendor \ symfony \ symfony \ src \ Symfony \ Component \ DependencyInjection \ Compiler \ ReplaceAliasByActualDefinitionPass.php：63
 C：\ projects \ myproject \ vendor \ symfony \ symfony \ src \ Symfony \ Component \ DependencyInjection \ Compiler \ ReplaceAliasByActualDefinitionPass.php：63
 C：\ projects \ myproject \ vendor \ symfony \ symfony \ src \ Symfony \ Component \ DependencyInjection \ Compiler \ ReplaceAliasByActualDefinitionPass.php：63
 C：\ projects \ myproject \ vendor \ symfony \ symfony \ src \ Symfony \ Component \ DependencyInjection \ Compiler \ ReplaceAliasByActualDefinitionPass.php：63
 C：\ projects \ myproject \ vendor \ symfony \ symfony \ src \ Symfony \ Component \ DependencyInjection \ Compiler \ ReplaceAliasByActualDefinitionPass.php：63
 C：\ projects \ myproject \ vendor \ symfony \ symfony \ src \ Symfony \ Component \ DependencyInjection \ Compiler \ ReplaceAliasByActualDefinitionPass.php：63
 C：\ projects \ myproject \ vendor \ symfony \ symfony \ src \ Symfony \ Component \ DependencyInjection \ Compiler \ ReplaceAliasByActualDefinitionPass.php：63
 C：\ projects \ myproject \ vendor \ symfony \ symfony \ src \ Symfony \ Component \ DependencyInjection \ Compiler \ ReplaceAliasByActualDefinitionPass.php：63
 C：\ projects \ myproject \ vendor \ symfony \ symfony \ src \ Symfony \ Component \ DependencyInjection \ Compiler \ ReplaceAliasByActualDefinitionPass.php：63
 C：\ projects \ myproject \ vendor \ symfony \ symfony \ src \ Symfony \ Component \ DependencyInjection \ Compiler \ ReplaceAliasByActualDefinitionPass.php：63
 C：\ projects \ myproject \ vendor \ symfony \ symfony \ src \ Symfony \ Component \ DependencyInjection \ Compiler \ ReplaceAliasByActualDefinitionPass.php：63
 C：\ projects \ myproject \ vendor \ symfony \ symfony \ src \ Symfony \ Component \ DependencyInjection \ Compiler \ ReplaceAliasByActualDefinitionPass.php：63
 C：\ projects \ myproject \ vendor \ symfony \ symfony \ src \ Symfony \ Component \ DependencyInjection \ Compiler \ ReplaceAliasByActualDefinitionPass.php：63
 C：\ projects \ myproject \ vendor \ symfony \ symfony \ src \ Symfony \ Component \ DependencyInjection \ Compiler \ ReplaceAliasByActualDefinitionPass.php：63
 C：\ projects \ myproject \ vendor \ symfony \ symfony \ src \ Symfony \ Component \ DependencyInjection \ Compiler \ Compiler.php：117
 C：\ projects \ myproject \ vendor \ symfony \ symfony \ src \ Symfony \ Component \ DependencyInjection \ ContainerBuilder.php：614
 C：\ projects \ myproject \ app \ bootstrap.php.cache：2565
 C：\ projects \ myproject \ app \ bootstrap.php.cache：2344
 C：\ projects \ myproject \ app \ AppKernel.php：43
 C：\ projects \ myproject \ vendor \ symfony \ symfony \ src \ Symfony \ Bundle \ FrameworkBundle \ Test \ KernelTestCase.php：141
 C：\ projects \ myproject \ vendor \ symfony \ symfony \ src \ Symfony \ Bundle \ FrameworkBundle \ Test \ WebTestCase.php：33
 C：\ projects \ myproject \ src \ mycompany \ myprojectBundle \ Tests \ Controller \ SomeControllerTest.php：23

此错误是什么意思，我需要更改什么才能在MySQL数据库上成功运行测试？

我想这并不是缺少MySQL驱动程序的迹象，因为我可以例如在命令行上对生产数据库很好地运行Symfony命令。