[ruby/json] Revert "Skip test failing with JRuby in CI"

This reverts commit https://github.com/ruby/json/commit/b7e1734d9ca0.

https://github.com/ruby/json/commit/5793694ee6

test/json/json_parser_test.rb

@@ -843,7 +843,6 @@ class JSONParserTest < Test::Unit::TestCase
 
   def test_frozen
     parser_config = JSON::Parser::Config.new({}).freeze
-    omit "JRuby failure in CI" if RUBY_ENGINE == "jruby"
     assert_raise FrozenError do
       parser_config.send(:initialize, {})
     end