CommandTester getDisplay has not output from monolog

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### Preconditions
Updated from 2.0.0-alpha8 to 2.0.0-alpha12 (it was present in 2.0.0-alpha10).

### Steps to reproduce

In a symfony 3.4 application I'm testing a command that receives a LoggerInterface with DI (it is a monolog logger handler with type console - https://symfony.com/doc/current/logging/monolog_console.html).
Via Monolog Bridge logs are sent to OutputInterface.

Handler is configured in this way:

```
qworker:
type: console
level: info
channels: ['qworker']
formatter: monolog.formatter.line
verbosity_levels:
VERBOSITY_NORMAL: info
```
In previous version I was testing in this way:

### Expected result

```
$output = $this->runCommand(
WorkerManagerConsumerCommand::NAME,
[
'data' => $this->encodePayload($data),
]
);

$this->assertContains('ERROR', $output);
```
### Actual result

In latest version (2.0.0-alpha-12) I have changed my code to this:

```
$commandTester = $this->runCommand(
WorkerManagerConsumerCommand::NAME,
[
'data' => $this->encodePayload($data),
]
);

$output = $commandTester->getDisplay();

$this->assertContains('ERROR', $output);
```

But the $output var now contains an empty string instead of the logged value.

In my integration test the output is always the same.

Start by reproducing the integration test described in the issue with Symfony's CommandTester, comparing runCommand behavior with getDisplay across the reported versions. Done means the command tester output contains the logged ERROR value instead of an empty string.

Written by the indexing model from the issue text.