Avoid DB changes after a request in budget spec

When using tests with a driver supporting JavaScript, there might be
concurrency issues if both the process running the test and the process
running the browser access the database once the browser has been
started.

spec/system/admin/budget_investments_spec.rb

@@ -529,7 +529,10 @@ describe "Admin budget investments", :admin do
 
       expect(page).to have_link "Calculate Winner Investments"
 
-      budget.update!(phase: "accepting")
+      select "Accepting projects", from: "Phase"
+      click_button "Update Budget"
+
+      expect(page).to have_content "Participatory budget updated successfully"
 
       visit admin_budget_budget_investments_path(budget)