Largo_Related tests and bugfixes

[benlk]

Changes

Largo_Related now uses the correct query to catch series landing pages.

Tests for Largo_Related's:

• ::__construct()
• ::popularity_sort()
• ::ids() in the following conditions:
  • unorganized series with a post published before the current post
  • unorganized series with a post published after the current post
  • a series with a landing page forcing the ascending sort order
  • a series with a landing page forcing the descending sort order
  • a series with a landing page forcing the custom sort order
  • a series with a landing page forcing descending sort order, with featured posts
  • a series with a landing page forcing ascending sort order, with featured posts
  • a category with posts
  • a tag
  • the current post matches posts in both categories and tags, with results drawn from both
  • no posts in the same series, categories, or tags, so only recent posts should be returned.
• ::cleanup_ids()

Why

Largo_Related bugfixes for #841, Test coverage for #782.

[benlk]

a series with a landing page forcing descending sort order, with featured posts
a series with a landing page forcing ascending sort order, with featured posts

These tests fail because the featured post is being returned last in the order, not first. They're marked incomplete. This may be because of a bug in the test, in Largo_Related, or in inc/wp-taxonomy-landing/functions/cftl-series-order.php