clean up failed_jobs before each spec

Change-Id: I31c75f6c13a987140a6ca6add51c1c780002b895
Reviewed-on: https://gerrit.instructure.com/4194
Reviewed-by: Brian Palmer <brianp@instructure.com>
Tested-by: Hudson <hudson@instructure.com>

vendor/plugins/delayed_job/spec_canvas/worker_spec.rb

@@ -11,6 +11,7 @@ describe Delayed::Worker do
   before(:each) do
     @worker = worker_create
     Delayed::Job.delete_all
+    Delayed::Job::Failed.delete_all
     SimpleJob.runs = 0
     Delayed::Worker.on_max_failures = nil
   end