PHP 5.2 Unit Tests no longer work due to deprecated PHPUnit PEAR channel

The PHP 5.2 build requires a phpunit and related DbUnit version that is compatible with PHP 5.2. Only phpunit < 3.7.x is compatible with PHP 5.2.
However, now that support for the pear installation method has ended we can no longer install phpunit using pear (https://github.com/sebastianbergmann/phpunit/wiki/End-of-Life-for-PEAR-Installation-Method).
Using composer for this is not possible as that will also require PHP >= 5.3.3.
This leaves us with the possibility of using a phar file for phpunit (https://phar.phpunit.de/).
Since there are no 3.6.x builds of phpunit available though, we should remove the PHP 5.2 build from the build configuration.