Add test to ensure WebTestCase does not trigger error

[nikophil]

I recently had a problem with WebTestCase, while I was testing #943 on a real life project

I needed to make this commit : 9425912

otherwise I had the infamous error:

LogicException: Booting the kernel before calling "Symfony\Bundle\FrameworkBundle\Test\WebTestCase::createClient()" is not supported, the kernel should only be booted once.

The scenario is the following:

• a test with the kernel runs
• tearDown() calls ensureKernelShutdown()
• AFTER THAT Factories::_shutdownFoundry() is called
• calling Configuration::instance() boots the kernel
• if the next test is a WebTestCase, it will throw

I'd like to do two things:

• now that priority for hook methods are a thing, we should add priorities to our hooks
• add a test which would prevent us if something like this happen in the future