Improve and reduce tests for get_blogs_of_user()

* Create half as many factory sites. See #30017
* Test the removal of a user from multiple sites.
* Expand tests to include second parameter for flagged sites.
* Remove duplicate test for deleted user.

Fixes #29996


git-svn-id: https://develop.svn.wordpress.org/trunk@29937 602fd350-edb4-49c9-b593-d223f7449a82