Improve layout of test for cached invalid, then valid site details

* Rename method to `test_get_blog_details_when_site_does_not_exist()`.
* Always assume `MAX(blog_id)` is 1 and therefore always create a burner.
* Remove tests specific to `wpmu_delete_blog()` and cache, to be handled elsewhere.
* Remove extra asertions.

See #30080, #23405

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