Add manual paging of results in swap-coauthors dry mode command

Must manually page, as the query is otherwise expecting the posts to be
modified and no longer match (so it's always querying page 1).

Fixes #260

php/class-wp-cli.php

@@ -503,6 +503,11 @@ public function swap_coauthors( $args, $assoc_args ) {
 				}
 			}
 
+			// In dry mode, we must manually advance the page
+			if ( $dry ) {
+				$query_args['paged']++;
+			}
+
 			$this->stop_the_insanity();
 
 			$posts = new WP_Query( $query_args );