Add timeout to infinite loop [Bug #8100]

On FreeBSD, it doesn't SEGV.
http://fbsd.rubyci.org/~chkbuild/ruby-trunk/log/20130323T170203Z.log.html.gz

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39894 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

KNOWNBUGS.rb

@@ -4,10 +4,13 @@
 #
 
 assert_normal_exit %q{
-  loop do
+  require 'timeout'
-    def x
+  timeout(2) do
-      "hello" * 1000
+    loop do
+      def x
+        "hello" * 1000
+      end
+      method(:x).call
     end
-    method(:x).call
   end
 }, '[ruby-core:53640] [Bug #8100]'