Added some missing model specs + PaperTrail session fix

spec/fabricators/api_user_fabricator.rb

@@ -1,6 +1,12 @@
+# default fabricator should be reusable
 Fabricator(:api_user) do
-  username 'gitlab'
+  username { sequence(:username) { |i| "username#{i}" } }
   password 'ghyt9e4fu'
   registrar
   active true
 end
+
+# use dedicated fabricator for fixed one
+Fabricator(:gitlab_api_user, from: :api_user) do
+  username 'gitlab'
+end