Fix system_test_example teardown

Original `after_teardown` must be called after the original `before_teardown` (which is added to `after` hooks by RSpec::Rails::MinitestLifecycleAdapter).

Fixes taking failures screenshot functionality (changed in rails/rails@ef12ccf).

Author: palkan

lib/rspec/rails/example/system_example_group.rb

@@ -89,7 +89,7 @@ def driven_by(*args, &blk)
           @routes = ::Rails.application.routes
         end
 
-        after do
+        append_after do
           orig_stdout = $stdout
           $stdout = StringIO.new
           begin