KNOWNBUGS.rb: assertion failure

* KNOWNBUGS.rb: add test for r52872, assertion failure.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@52879 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

KNOWNBUGS.rb

@@ -3,3 +3,12 @@
 # So all tests will cause failure.
 #
 
+assert_equal 'false', %q{
+  x = Object.new.taint
+  class << x
+    def to_s; "foo".freeze; end
+  end
+  x.taint
+  [x].join("")
+  eval '"foo".freeze.tainted?'
+}