Fix in models/exercise_spec.rb, repair average_percentage without submission

spec/spec_helper.rb

@@ -33,6 +33,11 @@ RSpec.configure do |config|
   config.filter_run :focus
   config.run_all_when_everything_filtered = true
 
+
+  #for --next-failure feature purpose
+  config.example_status_persistence_file_path = "examples.txt"
+  config.run_all_when_everything_filtered = true
+
   # Many RSpec users commonly either run the entire suite or an individual
   # file, and it's useful to allow more verbose output when running an
   # individual spec file.