Explicitly assign menu term relationship rather than piggybacking on wp_insert_post() with the tax_input argument.

That argument currently depends on user context (see #19373).

Adds unit test for properly updating orphaned menu items.

props danielbachhuber.
fixes #27113.


git-svn-id: https://develop.svn.wordpress.org/trunk@27556 602fd350-edb4-49c9-b593-d223f7449a82