StdErr output is after StdOut in step definition `the output should contain`

## Summary

I found that in version `1.0.0`, the contents of `StdOut` and `StdErr` do not keep their original sequence in step definitions like `the output should contain`. More specific, `StdErr` is after `StdOut`.

Version `0.14.x` is fine. So I think this should be a regression.

## Expected Behavior

The output of `StdErr` and `StdOut` should keep their original sequence in step definitions like `the output should contain`.

## Current Behavior

If the app prints some contents to `StdErr` first, and then to `StdOut`, like below:
```
Errors
Normal
```
We write the scenario according to the actual output.

However in step definitions like `the output should contain`, it will treat the output like:
```
Normal
Errors
```
Thus the comparison will fail.

## Possible Solution

Roll back to old versions.

## Steps to Reproduce (for bugs)

1. Try an CLI app which will print to `StdErr` before `StdOut`
2. Compose and run a BDD test case under version `1.0.0`
3. Compose and run a BDD test case under old versions, like `0.14.4`

## Context & Motivation

This will break CLI apps which mix `StdOut` and `StdErr` outputs (like put some warning messages between normal output)

## Your Environment

* Version used: 1.0.0
* Operating System and version: all
* Link to your project: N/A Sorry I could not provide it, since it's not an open source project.

Reproduce the mixed StdErr/StdOut case with Aruba 1.0.0 and 0.14.4, then locate the step definition for `the output should contain`. Trace how the two streams are combined and ensure the comparison preserves their original sequence; verify the behavior with a regression test.

Written by the indexing model from the issue text.