Remove PHP 5.6 tests

These tests with a specific version of PHP cannot run on "recent"
operating systems, and it does not really make sense to add logic to
conditionaly include them because according to [PHP:
Releases](https://www.php.net/releases/index.php):

> Support for PHP 5 has been discontinued since 10 Jan 2019.
> Please consider upgrading to 8.

Author: smortex