Use assert_separately to inspect test failure of TestJSONGenerate#test_broken_bignum

http://fbsd.rubyci.org/~chkbuild/ruby-trunk/log/20130617T010202Z.log.html.gz#test%2F-ext-

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41350 b2dd03c8-39d4-4d8f-98ff-823fe69b080e