Disable RSpec/NoExpectationExample for feature/system specs

[The GOV.UK Rails convention approach for feature/system specs](https://docs.publishing.service.gov.uk/manual/conventions-for-rails-applications.html#featuresystem-testing) have expectations within methods and thus falsely trigger this cop.

For example we think a test like this is valid:

```
  scenario do
    given_i_have_an_active_conversation_with_an_answered_question
    when_i_visit_the_conversation_page
    and_i_click_that_the_answer_was_useful
    then_i_see_that_my_feedback_was_submitted
  end

  def then_i_see_that_my_feedback_was_submitted
    expect(page).to have_content("Feedback submitted successfully.")
  end
```

Whereas this cop would trigger as there are no expectations in the
scenario.

config/rspec.yml

@@ -31,6 +31,13 @@ RSpec/InstanceVariable:
     - 'spec/features/**/*.rb'
     - 'spec/system/**/*.rb'
 
+# The GOV.UK Feature spec style has expectations in method calls so this
+# cop falsely triggers.
+RSpec/NoExpectationExample:
+  Exclude:
+    - 'spec/features/**/*.rb'
+    - 'spec/system/**/*.rb'
+
 # In GOV.UK we quite often test that a class received a method.
 RSpec/MessageSpies:
   Enabled: false
@@ -56,7 +63,7 @@ RSpec/ContextWording:
 # scenario.
 # We don't want this cop outside of feature or system specs though.
 RSpec/Dialect:
-  # Disables all Capybara-specific methods that have the same native 
+  # Disables all Capybara-specific methods that have the same native
   # RSpec method (e.g. are just aliases)
   PreferredMethods:
     background: :before