test: cover all UTF-8 fast-path aliases

Author: icecold009

lib/fs.js

@@ -3199,7 +3199,9 @@ function isUtf8Encoding(encoding) {
   return encoding === 'utf8' ||
     encoding === 'utf-8' ||
     encoding === 'UTF8' ||
-    encoding === 'UTF-8';
+    encoding === 'UTF-8' ||
+    encoding === 'Utf8' ||
+    encoding === 'Utf-8';
 }
 
 function encodeRealpathResult(result, options) {