Exception#backtrace may be nil if it is manually created

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@53142 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/lib/test/unit/parallel.rb

@@ -139,7 +139,7 @@ module Test
         rescue Errno::EPIPE
         rescue Exception => e
           begin
-            trace = e.backtrace
+            trace = e.backtrace || ['unknown method']
             err = ["#{trace.shift}: #{e.message} (#{e.class})"] + trace.map{|t| t.prepend("\t") }
 
             _report "bye", Marshal.dump(err.join("\n"))