[FrameworkBundle] Returns the kernel instance in KernelTestCase::bootKernel

See https://github.com/symfony/symfony/pull/22276

This is the new code allowed by this change:

```php
// Before
static::bootKernel($options);
$client = static::$kernel->getContainer()->get('test.client');

// after
$kernel = static::bootKernel($options)
$client = $kernel->getContainer()->get('test.client');
```

Not sure if we should mention or explain this.