Check symlink in tmpdir and do not use empty path

Author: znz

test/fileutils/test_fileutils.rb

@@ -51,7 +51,11 @@ def have_symlink?
     end
 
     def check_have_symlink?
-      File.symlink "", ""
+      Dir.mktmpdir do |dir|
+        Dir.chdir(dir) do
+          File.symlink "symlink", "symlink"
+        end
+      end
     rescue NotImplementedError, Errno::EACCES
       return false
     rescue