minor #3979 DX: enable php_unit_method_casing (keradus)

This PR was squashed before being merged into the 2.13-dev branch (closes #3979).

Discussion
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DX: enable php_unit_method_casing

Appendix for #3886

Commits
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3fff21a DX: enable php_unit_method_casing

Author: keradus

.php_cs.dist

@@ -58,6 +58,7 @@ $config = PhpCsFixer\Config::create()
         'ordered_class_elements' => true,
         'ordered_imports' => true,
         'php_unit_internal_class' => true,
+        'php_unit_method_casing' => true,
         'php_unit_ordered_covers' => true,
         'php_unit_set_up_tear_down_visibility' => true,
         'php_unit_strict' => true,

src/Fixer/PhpUnit/PhpUnitMethodCasingFixer.php

@@ -204,7 +204,7 @@ private function isMethod(Tokens $tokens, $index)
      */
     private function startsWith($needle, $haystack)
     {
-        return substr($haystack, 0, strlen($needle)) === $needle;
+        return substr($haystack, 0, \strlen($needle)) === $needle;
     }
 
     /**