Update individual term caches in `get_terms()`.

This was removed in [29915] as part of the attempt to add cache support to
`get_term_by()`. When that support was removed in [30900], it was not properly
restored.

This changeset includes a unit test to verify that the cache is properly primed
for terms found in `get_terms()`, as well as tests to verify the other cache
setting that was touched by [30900].

Fixes #30749. See #21760.
Built from https://develop.svn.wordpress.org/trunk@30954


git-svn-id: http://core.svn.wordpress.org/trunk@30942 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/taxonomy.php

@@ -1912,6 +1912,9 @@ function get_terms( $taxonomies, $args = '' ) {
 	}
 
 	$terms = $wpdb->get_results($query);
+	if ( 'all' == $_fields ) {
+		update_term_cache( $terms );
+	}
 
 	if ( empty($terms) ) {
 		wp_cache_add( $cache_key, array(), 'terms', DAY_IN_SECONDS );