[php-src] Issue #21374: Fix sapi/fpm/tests/proc-idle-timeout.phpt and remove XFAIL

[email protected] (iluuu1994)
Newsgroups php.bugs
Message-ID <[email protected]> 
Issue: https://github.com/php/php-src/issues/21374
Author: iluuu1994

### Description

See https://github.com/php/php-src/commit/3902f843f7ae33cf45a0a9acc4e30f4b92c7a6dd.

sapi/fpm/tests/proc-idle-timeout.phpt was marked as `XFAIL` because it fails multiple times in every nightly run. We should improve its stability and then remove the `XFAIL` again.

### PHP Version

```plain
8.2+
```

### Operating System

_No response_
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