Finish test coverage of Largo_Related

[benlk]

#842 didn't cover all the testable parts of the Largo_Related class when it was merged. Here's what's left to do:

• Largo_Related::ids() in the following conditions:
  • a series with a landing page forcing descending sort order, with featured posts (See Investigate reason for test_series_featured_desc and test_series_featured_asc failures #843)
  • a series with a landing page forcing ascending sort order, with featured posts (See Investigate reason for test_series_featured_desc and test_series_featured_asc failures #843)
  • 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.
• Largo_Related::cleanup_ids()

Regarding #843, I'm not sure what's going on with test_series_featured_asc and test_series_featured_desc. Either there's an incorrect assumption with how I wrote the function, or something else is up. But that's #843.

[benlk]

@rclations Should this be included in the work breaking out the Related Posts widget?

[rclations]

definitely - added a companion issue on that repo