…Nyholm)

This PR was squashed before being merged into the 5.3-dev branch.

Discussion
----------

[FrameworkBundle] Add KernelTestCase::getContainer()

| Q             | A
| ------------- | ---
| Branch?       | 5.x
| Bug fix?      | no
| New feature?  | yes
| Deprecations? | yes
| Tickets       |
| License       | MIT
| Doc PR        | symfony/symfony-docs#14731

There are at least 3 ways to get the container in a test class:

```php
class FooTest extends WebTestCase
{
    public function testGetContainerA()
    {
        $kernel = self::bootKernel();
        $container = $kernel->getContainer();
    }

    public function testGetContainerB()
    {
        self::bootKernel();
        $container = self::$container;
    }

    public function testGetContainerC()
    {
        $client = self::createClient();
        $container = $client->getContainer();
    }
}
```

I suggest to add a fourth =)

Basically, in tests you should always use the `test.service_container`. It is hard to remove A and C, but I can deprecate C and add a helper function.

```php
class FooTest extends WebTestCase
{
    public function testGetContainerTheOnlyWayYouShouldUse()
    {
        $container = $this->getContainer();
    }
}
```

This new way will also boot your kernel if it is not already booted.

Commits
-------

f4c97240ff [FrameworkBundle] Add KernelTestCase::getContainer()