Improve domain discard

- Extract rake task domain:discard
- Remove background job when keeping a domain

#790

test/models/domain/deletable_test.rb

@@ -5,10 +5,25 @@ class DomainDeletableTest < ActiveSupport::TestCase
     @domain = domains(:shop)
   end
 
-  def test_discard
-    refute @domain.discarded?
+  def test_discard_domain
     @domain.discard
     @domain.reload
+    assert QueJob.find_by("args->>0 = '#{@domain.id}'", job_class: DomainDeleteJob.name)
     assert @domain.discarded?
   end
+
+  def test_discard_invalid_domain
+    domain = domains(:invalid)
+    domain.discard
+    domain.reload
+    assert domain.discarded?, 'a domain should be discarded'
+  end
+
+  def test_keep_domain
+    @domain.discard
+    @domain.keep
+    @domain.reload
+    assert_nil QueJob.find_by("args->>0 = '#{@domain.id}'", job_class: DomainDeleteJob.name)
+    refute @domain.discarded?
+  end
 end