Remove redundant table alias check in WP_Meta_Query.

Also adds documentation for 'meta_query_find_compatible_table_alias' filter.

See #24093.

git-svn-id: https://develop.svn.wordpress.org/trunk@29953 602fd350-edb4-49c9-b593-d223f7449a82
Ensure that post types and taxonomies are reset between multisite tests.

This is a port of [29869] to multisite.xml.

See #29827.

git-svn-id: https://develop.svn.wordpress.org/trunk@29954 602fd350-edb4-49c9-b593-d223f7449a82