test_readline.rb: skip teardown failure

https://ci.appveyor.com/project/ruby/ruby/build/9271/job/e88212s136mr0dgr

I'll take a look at MSP-Greg's patch on readline later.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64369 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/readline/test_readline.rb

@@ -21,6 +21,9 @@ class TestReadline < Test::Unit::TestCase
   end
 
   def teardown
+    if ENV.key?('APPVEYOR') && /mingw/ =~ RUBY_PLATFORM
+      return
+    end
     ENV[INPUTRC] = @inputrc
     Readline.instance_variable_set("@completion_proc", nil)
     begin