In get_terms(), select term taxonomy count for all values of 'fields'.

Not having the count caused queries with 'fields' values of 'id=>name' and
'id=>slug' to return incorrect results when querying a hierarchical taxonomy
with 'hide_empty=true'.

Includes unit tests for get_terms() when using various combinations of 'fields',
'hide_empty', and 'hierarchical' arguments.

Props technical_mastermind.
Fixes #29859.

git-svn-id: https://develop.svn.wordpress.org/trunk@29845 602fd350-edb4-49c9-b593-d223f7449a82