Performance problem with container.dumper.inline_class_loader

I'v just migrated a project from Symfony 3.4 to 4.2. My test suite executed 5-6x longer. After investigation I discovered that parameter `container.dumper.inline_class_loader` with value `true` is the reason.

It's now a part of Flex recipe for FrameworkBundle: https://github.com/symfony/recipes/blob/master/symfony/framework-bundle/4.2/src/Kernel.php#L31

I think that it's worth consider to add information about that to documentation.

Start by reviewing the Symfony Flex recipe's Kernel.php reference and the existing documentation in LiipFunctionalTestBundle. Document the test-suite performance impact of container.dumper.inline_class_loader=true and explain the relevant configuration context; done when users can find and understand this warning.

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