Use KernelTestCase instead of WebTestCase for testing code only requiring the Container

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#9739)
| Applies to    | 2.5
| Fixed tickets | none

This documents using the new KernelTestCase introduced in symfony/symfony#9739, which was recently merged to master for 2.5-dev.

KernelTestCase was extracted from WebTestCase to become a more-focused base class for functional testing code that requires only a container but no client. WebTestCase is then more-focused to be used for tests requiring a client.

If you think this needs any kind of version callout in the docs to highlight the base class change we can add that too.

:rocket:

Previous PR #3244 against the 2.4 branch was closed because the corresponding feature was not merged into 2.4.