assertions.rb: defered message in FailDesc

* test/lib/test/unit/assertions.rb (FailDesc): allow to defer
  message generation, as well as other assertion methods.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57016 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/lib/test/unit/assertions.rb

@@ -530,6 +530,7 @@ EOT
             sigdesc << " (core dumped)"
           end
           full_message = ''
+          message = message.call if Proc === message
           if message and !message.empty?
             full_message << message << "\n"
           end