[FrameworkBundle] Add KernelTestCase::getContainer()

[Nyholm]

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:

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.

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

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

[wouterj]

At first I thought: Oh no. But I think this makes sense: you'll get a friendly error (instead of a completely different container) and no more "you'll have to boot the kernel first".

Sorry for the complete nitpicking on anything that wasn't PHP code in this review 🙈

[Nyholm]

Thank you for the review. I've updated the PR according to your suggestions.

[fabpot]

Thank you @Nyholm.

[Nyholm]

Thank you for merging.