Fix flaky investments order spec

We were clicking links and visiting pages without checking the previous
request had already finished. This might cause concurrent requests,
leading to unpredictable results.

It might be the reason why this test failed once when running our
continuous integration [1].

[1] https://github.com/consul/consul/runs/3295502777

spec/system/budgets/investments_spec.rb

@@ -297,8 +297,13 @@ describe "Budget Investments" do
       expect(order).not_to be_empty
 
       click_link "highest rated"
+
+      expect(page).to have_css "h2", exact_text: "highest rated"
+
       click_link "random"
 
+      expect(page).to have_css "h2", exact_text: "random"
+
       visit budget_investments_path(budget, heading_id: heading.id)
       new_order = all(".budget-investment h3").map(&:text)