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.

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git-svn-id: http://core.svn.wordpress.org/trunk@27399 1a063a9b-81f0-0310-95a4-ce76da25c4cd