Test 500 Internal server error page in system spec

We were testing what happens when users disable features in the admin
panel, so it makes sense to test what happens from the user's point of
view when trying to access a disabled feature: they see a page with the
test "Internal server error".

Whether we should responde with 500 Internal server error page or a 404
Not Found is up to debate; personally I find the latter more
appropriate.

Code based on the article "Changing Rails consider_all_requests_local in
RSpec fails" [1].

[1] http://atodorov.org/blog/2016/04/27/changing-rails-consider_all_requests_local-in-rspec-fails/

spec/spec_helper.rb

@@ -78,6 +78,18 @@ RSpec.configure do |config|
     sign_in(create(:administrator).user)
   end
 
+  config.before(:each, :show_exceptions) do
+    config = Rails.application.env_config
+
+    allow(Rails.application).to receive(:env_config) do
+      config.merge(
+        "action_dispatch.show_exceptions" => true,
+        "action_dispatch.show_detailed_exceptions" => false,
+        "consider_all_requests_local" => false
+      )
+    end
+  end
+
   config.before(:each, :delay_jobs) do
     Delayed::Worker.delay_jobs = true
   end

spec/system/admin/feature_flags_spec.rb

@@ -14,7 +14,7 @@ describe "Admin feature flags", :admin do
     end
   end
 
-  scenario "Disable a participatory process" do
+  scenario "Disable a participatory process", :show_exceptions do
     setting = Setting.find_by(key: "process.budgets")
     budget = create(:budget)
 
@@ -33,8 +33,14 @@ describe "Admin feature flags", :admin do
       expect(page).not_to have_link "Participatory budgets"
     end
 
-    expect { visit budget_path(budget) }.to raise_exception(FeatureFlags::FeatureDisabled)
-    expect { visit admin_budgets_path }.to raise_exception(FeatureFlags::FeatureDisabled)
+    visit budget_path(budget)
+
+    expect(page).to have_content "Internal server error"
+
+    visit admin_budgets_path
+
+    expect(page).to have_current_path admin_budgets_path
+    expect(page).to have_content "Internal server error"
   end
 
   scenario "Enable a disabled participatory process" do