Ordering by `RAND()`:

The shortcode callbacks for `gallery` and `playlist` check for `'RAND' == $atts['order']`, which isn't a valid value for `order`. Remove those checks and update the docs.

In `WP_Query`, if the value of `orderby` is `rand`, `order` is irrelevant and should be unset.

Adds unit tests.

Fixes #29629.

Built from https://develop.svn.wordpress.org/trunk@29760


git-svn-id: http://core.svn.wordpress.org/trunk@29532 1a063a9b-81f0-0310-95a4-ce76da25c4cd