Fix random failure on system test with ajax

If application has ajax, browser may begin request after rollback.
`teardown_fixtures` will be called after `super` on `after_teardown`
so we must call `Capybara.reset_sessions!` before `super`

b61a56541a/activerecord/lib/active_record/fixtures.rb (L857)

actionpack/lib/action_dispatch/system_testing/test_helpers/setup_and_teardown.rb

@@ -11,8 +11,8 @@ module ActionDispatch
 
         def after_teardown
           take_failed_screenshot
-          super
           Capybara.reset_sessions!
+          super
         end
       end
     end

railties/test/application/test_runner_test.rb

@@ -576,6 +576,34 @@ module ApplicationTests
         capture(:stderr) { run_test_command("test/models/warnings_test.rb -w") })
     end
 
+    def test_reset_sessions_before_rollback_on_system_tests
+      app_file "test/system/reset_session_before_rollback_test.rb", <<-RUBY
+        require "application_system_test_case"
+
+        class ResetSessionBeforeRollbackTest < ApplicationSystemTestCase
+          def teardown_fixtures
+            puts "rollback"
+            super
+          end
+
+          Capybara.singleton_class.prepend(Module.new do
+            def reset_sessions!
+              puts "reset sessions"
+              super
+            end
+          end)
+
+          test "dummy" do
+          end
+        end
+      RUBY
+
+      run_test_command("test/system/reset_session_before_rollback_test.rb").tap do |output|
+        assert_match "reset sessions\nrollback", output
+        assert_match "1 runs, 0 assertions, 0 failures, 0 errors, 0 skips", output
+      end
+    end
+
     private
       def run_test_command(arguments = "test/unit/test_test.rb")
         Dir.chdir(app_path) { `bin/rails t #{arguments}` }