Do not leave test file

Run this test separately because something seems remained unreleased
on Windows.

test/ruby/test_io_buffer.rb

@@ -519,12 +519,14 @@ class TestIOBuffer < Test::Unit::TestCase
   end
 
   def test_private
-    tmpdir = Dir.tmpdir
-    buffer_path = File.join(tmpdir, "buffer.txt")
-    File.write(buffer_path, "Hello World")
-
-    File.open(buffer_path) do |file|
+    Tempfile.create(%w"buffer .txt") do |file|
+      file.write("Hello World")
+      file.close
+      assert_separately(["-W0", "-", file.path], "#{<<-"begin;"}\n#{<<-'end;'}")
+      begin;
+        file = File.open(ARGV[0], "r+")
         buffer = IO::Buffer.map(file, nil, 0, IO::Buffer::PRIVATE)
+        begin
           assert buffer.private?
           refute buffer.readonly?
 
@@ -536,5 +538,7 @@ class TestIOBuffer < Test::Unit::TestCase
         ensure
           buffer&.free
         end
+      end;
+    end
   end
 end