Simplify tests checking dates are present

When we create a record like a debate or an event and we check the page
content, we want to make sure that today's date is present, since it's
the date where the record is supposed to have been created.

This way we avoid querying the database after the browser has been
started.

spec/system/admin/stats_spec.rb

@@ -216,7 +216,7 @@ describe "Stats", :admin do
   end
 
   context "graphs" do
-    scenario "event graphs" do
+    scenario "event graphs", :with_frozen_time do
       campaign = create(:campaign)
 
       visit root_path(track_id: campaign.track_id)
@@ -227,9 +227,9 @@ describe "Stats", :admin do
       end
 
       expect(page).to have_content "#{campaign.name} (1)"
+
       within("#graph") do
-        event_created_at = Ahoy::Event.find_by(name: campaign.name).time
-        expect(page).to have_content event_created_at.strftime("%Y-%m-%d")
+        expect(page).to have_content Date.current.strftime("%Y-%m-%d")
       end
     end
   end