Introduce `author__in` and `author__not_in` query vars. Fixes issue with multiple author exclusion when comma-separated string is passed for `author`. Adds a bunch of missing unit tests.

Props pollett for initial patch.
Fixes #16854.



git-svn-id: https://develop.svn.wordpress.org/trunk@25248 602fd350-edb4-49c9-b593-d223f7449a82