Kernel not cleaned up in getContainer()

Running several [`WebTestCase`](https://github.com/liip/LiipFunctionalTestBundle/blob/master/Test/WebTestCase.php) descendants leads to `UnexpectedValueException: RecursiveDirectoryIterator::__construct(...): failed to open dir: Too many open files`. Perhaps this is due to kernel not being reused correctly?

Start with Test/WebTestCase.php and inspect the getContainer() entry point, then reproduce the failure by running several WebTestCase descendants. Trace how the kernel is handled across those runs and verify that repeated tests no longer lead to the reported Too many open files exception.

Written by the indexing model from the issue text.