[2.0] Allow to mock services when running command

**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` ?)

Start by reading the existing runCommand method, including its reuseKernel option, and the createClient and HttpClient entry points. Compare how the test client exposes the container before execution; done means commands can run with services replaced by mocks without requiring direct kernel manipulation.

Written by the indexing model from the issue text.