Allow running commands with existing kernel

When running `WebTestCase::runCommand()` it always boots a new kernel. 
There are use cases where reusing a `Kernel` is required to test some functionality.
This patch adds a non-BC parameter which reuses a kernel from `WebTestCase::getContainer()`:

```php 
$this->runCommand('command:name', [ 'params' ], $reuseKernel = true);
```

This is loosely related to #111 but whereas the PR allows us to access the container **after** running the command, this PR allows ups to access the container **before** running the command.