In wp_insert_term(), allow a term with an existing name if a unique $slug has been provided.

`wp_insert_term()` protects against the creation of terms with duplicate names
at the same level of a taxonomy hierarchy. However, it's historically been
possible to override this protection by explicitly providing a value of `$slug`
that is unique at the hierarchy tier. This ability was broken in [31734], and
the current changeset restores the original behavior.

A number of unit tests are added and refactored in support of these changes.

See #17689 for discussion of a fix that was superceded by [31734]. This commit
retains the fix for the underlying bug described in that ticket.

See #31328.
Built from https://develop.svn.wordpress.org/trunk@31792


git-svn-id: http://core.svn.wordpress.org/trunk@31774 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.2-beta1-31791';
+$wp_version = '4.2-beta1-31792';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.