Fixed tests for php5.4+, data change for post_(multi)select fields

- post select and multiselect fields now contain post slugs in the
  html rather than post IDs. This is to make lookups through an API
  where post IDs may not be reliable, or predictable, easier. When
  using RAMP for example to move to production, IDs on the production
  server will mismatch those from the staging server but slugs will
  remain.