test_fileutils.rb: improve my_rm_rf

* test/fileutils/test_fileutils.rb (my_rm_rf): get rid of special
  chars and remove by rmdir on Windows.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@51690 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/fileutils/test_fileutils.rb

@@ -123,7 +123,9 @@ class TestFileUtils < Test::Unit::TestCase
 
   def my_rm_rf(path)
     if File.exist?('/bin/rm')
-      system %Q[/bin/rm -rf "#{path}"]
+      system "/bin/rm", "-rf", path
+    elsif /mswin|mingw/ =~ RUBY_PLATFORM
+      system "rmdir", "/q/s", path.gsub('/', '\\'), err: IO::NULL
     else
       FileUtils.rm_rf path
     end