LanguageNameIndexer/Search: use unicode aware lowercasing

With this MEÄNKELI with typos=1 finds results.

Updated test case for lowercased result. Renamed variables in test
file for clarity. Updated the default value for MW_INSTALL_PATH to
work with the default layout.

Change-Id: Id93c84d308705f55b4d2378fc8c7b7f243e1b53f

data/LanguageNameIndexer.php

@@ -37,7 +37,7 @@ class LanguageNameIndexer extends Maintenance {
 		foreach ( $languages as $sourceLanguage => $autonym ) {
 			$translations = LanguageNames::getNames( $sourceLanguage, 0, 2 );
 			foreach ( $translations as $targetLanguage => $translation ) {
-				$translation = strtolower( $translation );
+				$translation = mb_strtolower( $translation );
 				$bucket = LanguageNameSearch::getIndex( $translation );
 				$buckets[$bucket][$translation] = $targetLanguage;
 			}