Needs test coverage

[karlwancl]

Needs test codes to provide better test coverage

[karlwancl]

Some tests are added via the commit 1a998f105f6b768af463ed97d092b591ab490cea The followings are still not covered:

• Fetch & Download print jobs
• Submit print jobs
• Share printer to users
• Unshare printer from users
• Xmpp print job notification

[karlwancl]

This issue is closed now, will come back if there's a need for test completion