Invalidate the post cache for posts associated with a user who has been removed from a blog in `remove_user_from_blog()`. Adds a unit test.

Props nprasath002 for the initial patch.
Fixes #25545.



git-svn-id: https://develop.svn.wordpress.org/trunk@27087 602fd350-edb4-49c9-b593-d223f7449a82