test_jit.rb: show debug info for success_count test

to investigate
https://rubyci.org/logs/rubyci.s3.amazonaws.com/unstable11s/ruby-trunk/log/20181221T092505Z.fail.html.gz

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66479 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_jit.rb

@@ -877,10 +877,8 @@ class TestJIT < Test::Unit::TestCase
         Process.waitpid(pid)
       end;
       success_count = err.scan(/^#{JIT_SUCCESS_PREFIX}:/).size
-      assert_equal(3, success_count)
-
-      lines = err.lines
       debug_info = "stdout:\n```\n#{out}\n```\n\nstderr:\n```\n#{err}```\n"
+      assert_equal(3, success_count, debug_info)
 
       # assert no remove error
       assert_equal("Successful MJIT finish\n" * 2, err.gsub(/^#{JIT_SUCCESS_PREFIX}:[^\n]+\n/, ''), debug_info)