Add tests for save_meta method

[Nikschavan]

Is your feature request related to a problem? Please describe.

unlist-posts/class-unlist-posts-admin.php

Line 109 in d3e1721

public function save_meta( $post_id ) {

Verify that the function does add and remove the post id from the option unlist_posts, does not add duplicate post ids to the option, User needs to have capability of edit_post in order to set the option.