[2.0] Allow to mock services when running command

[magnetik]

Is your feature request related to a problem? Please describe.
I'm "mocking" some services when using the test client. It's something like that:

$client = $this->getClient();
$clientMock = $this->getMockBuilder(Client::class)
		->disableOriginalConstructor()
		->getMock();

$clientMock
	->method('doThings')
	->willReturn(true);

$client->getContainer()->set(Client::class, $clientMock);

Describe the solution you'd like
Instead of running the command directly, a CommandClient could be created (that could be like the "HttpClient", that allows to run any command and access the container before.

Describe alternatives you've considered
Maybe instead of having the ability to reuse the current kernel in protected function runCommand(string $name, array $params = [], bool $reuseKernel = false), it could be useful to provide your own kernel (and use a kernel from createClient ?)

[Jean85]

I fear that that approach could be flawed. The reason for reusing the kernel is that it's already booted, so any edit is not lost before launching the command.

How would you suggest to pass a different Kernel? Wouldn't be dangerous, due to differences in respect to the system under test?

[magnetik]

Indeed I prefer the first approach (having a CommandClient that allow container modification).