ApiTestCase can lead to several kernels (and therefore several entity managers and so on)

[Niolak]

API Platform version(s) affected: 3.2.14

Description
ApiTestCase::createClient always creates a new kernel. If you need services in your test to create the context with static::getContainer()->get(MyService::class);, two kernels will be created, which leads to several entity managers.

How to reproduce

class TransmissionActionTest extends ApiTestCase
{
    public function testSomething(): void
    {
      $myService = static::getContainer()->get(MyService::class); // Here, the kernel is booted via getContainer
      $myService->createAContextForTheTest();
      
      $response = static::createClient()->request('POST', '/my-action'); // Here, a 2nd kernel is created
      
      // example of what could go wrong next
      $myRepo = static::getContainer()->get(MyResourceRepository::class);
      $resourceCreatedByTheAction = $myRepo->findOneBy(['created-by-my-action' => true]); // not the same entitymanager as the one used by the request
      $this->assertNotNull($resourceCreatedByTheAction); // failure
    }
}

(I've simplified the example for readability: I'm not sure that findOneBy would return null on such a simple example. The fact remains that the wrong entitymanager is used, which can be a problem in some cases.)

Possible Solution
In ApiTestCase::createClient(), create the kernel only if not booted.
Replace

$kernel = static::bootKernel($kernelOptions);

by

$kernel = static::$booted ? static::$kernel : static::bootKernel($kernelOptions);

I even think that createClient should not create the kernel (dependency injection principle) but then the change would be a breaking change.