More increae timeout.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@35286 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/json/test_json_generate.rb

@@ -221,7 +221,7 @@ EOT
     GC.start
     stress, GC.stress = GC.stress, true
 
-    timeout(5) do
+    timeout(30) do
       10.times do |i|
         tmp = bignum_too_long_to_embed_as_string.to_json
         assert_equal expect, tmp