Add extra step in incompatible investments test

The test was always passing because incompatible investments are only
shown after clicking on the "Show all" link. So now we're clicking the
link  before checking the "incompatibles" table isn't shown.

I don't think "incompatibles" is a valid English word; for now, I'm
leaving it like this until a native English speaker reviews it.

spec/system/budgets/results_spec.rb

@@ -155,6 +155,14 @@ describe "Results" do
     visit budget_path(budget)
     click_link "See results"
 
+    expect(page).to have_content investment1.title
+    expect(page).to have_content investment2.title
+    expect(page).to have_content investment3.title
+    expect(page).not_to have_content investment4.title
+
+    click_link "Show all"
+
+    expect(page).to have_content investment4.title
     expect(page).not_to have_content "Incompatibles"
   end
 end