Create less headings in budget investment tests

We don't need to create a heading every time we create an investment; we
can use an existing one by default.

Some tests are now much faster and don't fail on Travis due to reaching
Capybara's timeout anymore.

spec/factories/budgets.rb

@@ -83,7 +83,8 @@ FactoryBot.define do
 
   factory :budget_investment, class: "Budget::Investment" do
     sequence(:title) { |n| "Budget Investment #{n} title" }
-    heading { association :budget_heading, budget: budget }
+    heading { budget&.headings&.reload&.sample || association(:budget_heading, budget: budget) }
+
     association :author, factory: :user
     description          { "Spend money on this" }
     price                { 10 }

spec/models/budget/stats_spec.rb

@@ -87,7 +87,7 @@ describe Budget::Stats do
 
     it "doesn't count nil user ids" do
       create(:budget_ballot_line, investment: investment,
-        ballot: create(:budget_ballot, budget: budget, user: nil, physical: true)
+        ballot: create(:budget_ballot, budget: budget.reload, user: nil, physical: true)
       )
 
       expect(stats.total_participants_vote_phase).to be 0