jit_support.rb: debug check_support stderr

rubyci freebsd11zfs doesn't pass this check. I want to know the stderr.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63507 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/lib/jit_support.rb

@@ -19,7 +19,11 @@ module JITSupport
       $stderr.puts "TestJIT: #jit_supported? check timed out"
       false
     else
-      err.match?(JIT_SUCCESS_PREFIX)
+      err.match?(JIT_SUCCESS_PREFIX).tap do |success|
+        unless success
+          $stderr.puts "TestJIT.check_support stderr:\n```\n#{err}\n```\n"
+        end
+      end
     end
   end