Use pathinfo() in AutoLoaderStructureTest::testPSR4Completeness

Use pathinfo() in AutoLoaderStructureTest::testPSR4Completeness

When setting AutoloadNamespaces to './' in extension.json, the test
AutoLoaderStructureTest::testPSR4Completeness would fail. The directory
path is not made canonical while the file is, which causes the substr()
call being used to strip too many characters. For example:

  $dir : /mediawiki/extensions/Wikidata.org/./
  $file: /mediawiki/extensions/Wikidata.org/Hooks.php

$abbrFileName = substr( substr( $file, strlen( $dir ) ), 0, -4 );
>>> oks

Use pathinfo() to parse the filename. Yields 'Hooks' as expected.

Bug: T198077
Change-Id: Ia8a11d87788b32ddb426a16a61b410b05ff5f15e