Fixed leaked file descriptor

  Leaked file descriptor: TestRDocGeneratorDarkfish#test_template_stylesheets: 11 : #<File:./tmp20210913-30865-7c4ey8/hoge20210913-30865-evm2uo.css>

ruby/ruby@ec6b444

Author: hsbt

test/rdoc/test_rdoc_generator_darkfish.rb

@@ -223,6 +223,7 @@ def test_generated_method_with_html_tag_yield
   def test_template_stylesheets
     css = Tempfile.create(%W'hoge .css', Dir.mktmpdir('tmp', '.'))
     File.write(css, '')
+    css.close
     base = File.basename(css)
     refute_file(base)