Ensure submission is complete before showing answers

Fixes CNVS-18802

Test plan
 - Create a quiz with multiple attempts and 'Let Students See The
 Correct Answers' 'Only After Their Last Attempt'
 - Take quiz untill you are on your final attempt
 - Answers should not be visible during final attempt
 - When final attempt is completed answers should now be visible

Change-Id: Ibf729afbe491b958d58b6b813379c9b40e678088
Reviewed-on: https://gerrit.instructure.com/49507
Tested-by: Jenkins
Reviewed-by: Ryan Taylor <rtaylor@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Brian Finney <bfinney@instructure.com>

app/models/quizzes/quiz.rb

@@ -353,7 +353,7 @@ class Quizzes::Quiz < ActiveRecord::Base
     return false unless self.show_correct_answers
 
     if user.present? && self.show_correct_answers_last_attempt && quiz_submission = user.quiz_submissions.where(quiz_id: self.id).first
-      return quiz_submission.attempts_left == 0
+      return quiz_submission.attempts_left == 0 && quiz_submission.complete?
     end
 
     # If we're showing the results only one time, and are letting students

spec/models/quizzes/quiz_spec.rb

@@ -1432,17 +1432,23 @@ describe Quizzes::Quiz do
     context "show_correct_answers_last_attempt is true" do
       let(:user) { User.create! }
 
-      it "shows the correct answers on last attempt" do
+      it "shows the correct answers on last attempt completed" do
         quiz = @course.quizzes.create!({
           title: 'test quiz',
           show_correct_answers: true,
           show_correct_answers_last_attempt: true,
-          allowed_attempts: 1
+          allowed_attempts: 2
         })
 
         quiz.publish!
 
         submission = quiz.generate_submission(user)
+        expect(quiz.show_correct_answers?(user, submission)).to be_falsey
+        submission.complete!
+
+        submission = quiz.generate_submission(user)
+        expect(quiz.show_correct_answers?(user, submission)).to be_falsey
+        submission.complete!
 
         expect(quiz.show_correct_answers?(user, submission)).to be_truthy
       end