Fix a coding standard mistake about phpunit $this vs self::

Found by phpstan

Signed-off-by: William Desportes <williamdes@wdes.fr>

Author: williamdes

tests/Misc/TranslatorTest.php

@@ -30,7 +30,7 @@ public function setUp(): void
             return;
         }
 
-        $this->markTestSkipped('The phpmyadmin/motranslator package is missing.');
+        self::markTestSkipped('The phpmyadmin/motranslator package is missing.');
     }
 
     public static function tearDownAfterClass(): void