test(user): add test for user deletion job

- User Deletion Job can be queued
- it will also delete one user after successful deletion
- it will send deleter an email should the deletion is unsuccessful

spec/jobs/course/user_deletion_job_spec.rb

@@ -0,0 +1,42 @@
+# frozen_string_literal: true
+require 'rails_helper'
+
+RSpec.describe Course::UserDeletionJob do
+  let(:instance) { Instance.default }
+  with_tenant(:instance) do
+    let!(:course) { create(:course) }
+    let!(:user) { create(:course_manager, course: course).user }
+    let!(:student) { create(:course_student, course: course) }
+    subject { Course::UserDeletionJob }
+
+    before do
+      subject.instance_variable_set(:@course, course)
+      subject.instance_variable_set(:@course_user, student)
+      subject.instance_variable_set(:@current_user, user)
+    end
+
+    it 'can be queued' do
+      expect { subject.perform_later(course, student, user) }.
+        to have_enqueued_job(subject).exactly(:once)
+    end
+
+    it 'delete one user upon successful course user deletion' do
+      expect { subject.perform_now(course, student, user) }.
+        to change { course.course_users.count }.by(-1)
+    end
+
+    it 'sends failure email upon failing course user deletion' do
+      allow(student).to receive(:destroy).and_return(false)
+
+      deletion_job = subject.perform_later(course, student, user)
+      deletion_job.perform_now
+
+      emails = ActionMailer::Base.deliveries.map(&:to).map(&:first)
+      email_subjects = ActionMailer::Base.deliveries.map(&:subject)
+
+      expect(ActionMailer::Base.deliveries.count).to eq(1)
+      expect(emails).to include(user.email)
+      expect(email_subjects).to include('course.mailer.course_user_deletion_failed_email.subject')
+    end
+  end
+end