Revert "Use pathinfo() in AutoLoaderStructureTest::testPSR4Completeness"

Revert "Use pathinfo() in AutoLoaderStructureTest::testPSR4Completeness"

This reverts commit 634c2ec2afd93827e5aaf571396eb76d35948188.

Reason for revert: that strips the PSR4 directories from the class.

Example:
$dir BlueSpiceFoundation/src/"
$file BlueSpiceFoundation/src/ConfigDefinition/IntSetting.php

$abbrFileName: IntSetting
$expectedClassName: BlueSpice\IntSetting

$abbrFileName should be relative to $dir and not just the filename.

Bug: T198077
Change-Id: Ie934e309fee0392439b4e26d86249f0650e5ea67