In #6340 I found that one of our tests tries to clear the jruby.home property and then require "rbconfig". However we load the rbconfig extension via rbconfig.rb here: https://github.com/jruby/jruby/blob/master/lib/ruby/stdlib/rbconfig.rb

There may be other built-in libraries that depend on a single load_ext in stdlib.

We should consider moving such files into the base jruby.jar file, since they are frequently necessary for normal operation of Ruby, with or without a standard library present.