cleanup for #18586 (modeltests.basic.model test.test object creation)

https://code.djangoproject.com/ticket/18586

Split up 300 line test method (inherited from doctest days)

* Extracted method's relevant test assertions into multiple test methods on new class ModelInstanceCreationTests
* moved the extra assertions from the test method as separate test methods to the relevant existing testclasses where possible, created new ones where couldn't find an appropriate one
* Test data was changed to make the new tests more readable in isolation and to accommodate the various different Article models across the various test folders

I left the PR as multiple commits to be able to find refactoring mistakes while the PR is being reviewed, can be squashed later.