Expand tests around wpmu_delete_blog()

* Test cache after a site is deleted or flagged as deleted.
* Test tables after a site is deleted or flagged as deleted.
* Test tables and cache after the main site is deleted.
* Test site count after a site is deleted or flagged as deleted.

See #30080


git-svn-id: https://develop.svn.wordpress.org/trunk@30106 602fd350-edb4-49c9-b593-d223f7449a82