Fix flaky spec in budget supports admin stats

The test that was not passing sometimes since commit 915999db4 because
we no longer created the budget before visiting the page that loads it.
So now we're forcing its creation with `let!`.

spec/system/admin/stats_spec.rb

@@ -88,7 +88,7 @@ describe "Stats", :admin do
 
   describe "Budget investments" do
     context "Supporting phase" do
-      let(:budget) { create(:budget) }
+      let!(:budget) { create(:budget) }
       let(:heading_all_city) { create(:budget_heading, budget: budget) }
 
       scenario "Number of users and supports in investment projects" do