Create tests for `orders` app

[Ramin-RX7]

For the orders app create these tests:

• models
• urls
• views
• admin panel
• forms

Few notes:

1. Do not test multiple parts of the app in the same test
2. Have the coverage of 100% on your this app
3. Make sure you test everything possible (response status code, template used, context, model fields, model methods, view response contents, etc.)
4. Make a package for tests and put different test cases into their own corresponding file.