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

[javiereguiluz]

See symfony/symfony#22276

This is the new code allowed by this change:

// 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.

[xabbuh]

We could update the existing code examples.