spec: fix flakey grade weight notification spec

It appears that this problem is that more than one NotificationPolicy
was being created, however one of them is not fetchable for some reason.

Before this patchset, the issue was surfaced by the following:

    rspec './spec/models/course_spec.rb[28:3:2,28:4:1]' --seed 37829

Test plan:
- run the command above

Change-Id: I55ae0f21e22571dfd178a59a1d5a7385fa60c55b
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/233464
Tested-by: Service Cloud Jenkins <[email protected]>
Reviewed-by: James Williams <[email protected]>
QA-Review: Derek Bender <[email protected]>
Product-Review: Derek Bender <[email protected]>

spec/models/course_spec.rb

@@ -5233,9 +5233,10 @@ def new_external_tool(context)
 
   describe 'grade weight notification' do
     before :once do
-      course_with_student(:active_all => true, :active_cc => true)
+      course_with_student(:active_all => true)
+      @student.communication_channels.create!(:path => '[email protected]').confirm!
       n = Notification.create!(name: 'Grade Weight Changed', category: 'TestImmediately')
-      NotificationPolicy.create(:notification => n, :communication_channel => @student.communication_channel, :frequency => "immediately")
+      NotificationPolicy.create!(:notification => n, :communication_channel => @student.communication_channel, :frequency => "immediately")
     end
 
     it "sends a notification when the course scheme changes" do