minor #16107 Fix assert statement (nacholibre)

This PR was squashed before being merged into the 4.4 branch.

Discussion
----------

Fix assert statement

In my local version of phpunit 9.5.9 `$this->assertContains` has the following signature:
```php
public static function assertContains($needle, iterable $haystack, string $message = ''): void
{
    //..
}
```

Where `$commandTester->getDisplay();` returns string and the test fails with

```
TypeError: Argument 2 passed to PHPUnit\Framework\Assert::assertContains() must be iterable, string given, called in..
```

Commits
-------

37d1373 Fix assert statement

Author: javiereguiluz

console.rst

@@ -376,7 +376,7 @@ console::
 
             // the output of the command in the console
             $output = $commandTester->getDisplay();
-            $this->assertContains('Username: Wouter', $output);
+            $this->assertStringContainsString('Username: Wouter', $output);
 
             // ...
         }