Revert "Skip test failing with JRuby in CI"

This reverts commit b7e1734.

Author: etiennebarrie

test/json/json_parser_test.rb

@@ -843,7 +843,6 @@ def test_parse_whitespace_after_newline
 
   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