Fix system spec spec on Rails 7

ActionPack is checking `instance_created?` now, and it was `false`, so
`take_screenshot` was not called.

```
def take_failed_screenshot
  take_screenshot if failed? && supports_screenshot? && Capybara::Session.instance_created?
end
```

rails/rails@1dfcffb

Author: pirj

spec/rspec/rails/example/system_example_group_spec.rb

@@ -72,6 +72,7 @@ module RSpec::Rails
 
     describe '#after' do
       it 'sets the :extra_failure_lines metadata to an array of STDOUT lines' do
+        allow(Capybara::Session).to receive(:instance_created?).and_return(true)
         group = RSpec::Core::ExampleGroup.describe do
           include SystemExampleGroup