Use Zeitwerk::Registry to unload loaders in tests

Previously I enabled ObjectSpace in JRuby in zeitwerk_helpers.rb
programatically, but it did not fully work.
ObjectSpace.each_object(Zeitwerk::Loader) kept returning empty list. I
suppose this was because the ObjectSpace was enabled too late. It worked
correctly when JRUBY_OPTS="-X+O" wever passed in the command line.

However, because the CI config is set via repo automation, it was hard
to configure it just for dry-systems and would have a performance
penalty if we enabled it for all gems.

So instead, I figured out a way to drop usage of ObjectSpace in favour
of Zeitwer's Registry module, which does the same thing (presumably even
better).

Author: katafrakt

spec/support/zeitwerk_helpers.rb

@@ -1,13 +1,8 @@
 # frozen_string_literal: true
 
-if RUBY_ENGINE == "jruby"
-  require "jruby"
-  JRuby.objectspace = true
-end
-
 module ZeitwerkHelpers
   def teardown_zeitwerk
-    ObjectSpace.each_object(Zeitwerk::Loader) do |loader|
+    Zeitwerk::Registry.loaders.each do |loader|
       if loader.dirs.any? { |dir| dir.include?("/spec/") || dir.include?(Dir.tmpdir) }
         loader.unregister
       end