limit candidate quiz dropdown to only show "active" quizzes

completes #105

test/fixtures/quizzes.yml

@@ -14,3 +14,8 @@ studio:
   name: Studio Screening One
   unit: PD Studio
   dept: FED
+
+empty_quiz:
+  name: A quiz with no questions
+  unit: PD
+  dept: FED

test/models/quiz_test.rb

@@ -2,7 +2,10 @@
 require 'test_helper'
 
 class QuizTest < ActiveSupport::TestCase
-  # test "the truth" do
-  #   assert true
-  # end
+  test "all_active only returns quizzes with questions" do
+    empty_quiz = quizzes(:empty_quiz)
+    quizzes = Quiz.all_active
+
+    refute quizzes.any? { |q| q.id == empty_quiz.id }
+  end
 end