default result sorting to quiz submission descending

completes #85

test/fixtures/candidates.yml

@@ -57,6 +57,7 @@ richard: # Richard has completed AND submitted the test
   recruiter: recruiter
   quiz: fed
   completed: true
+  completed_at: <%= DateTime.current %>
   reminded: false
   test_hash: 6NjnourLE6Y
   review_status: 1
@@ -78,6 +79,7 @@ stacy: # Stacy has completed AND submitted the test
   recruiter: recruiter
   quiz: fed
   completed: true
+  completed_at: <%= DateTime.current %>
   reminded: false
   test_hash: s6oFExZliYYFx
   review_status: 2
@@ -89,6 +91,7 @@ henry: # Henry has completed AND submitted the test
   recruiter: recruiter
   quiz: fed
   completed: true
+  completed_at: <%= DateTime.current %>
   reminded: false
   test_hash: egPomAuVDeCEp
 
@@ -99,6 +102,7 @@ wade: # Wade has completed AND submitted the test
   recruiter: recruiter
   quiz: fed
   completed: true
+  completed_at: <%= DateTime.current %>
   reminded: false
   test_hash: BkSkpapJnkz2N