Unit tests for unified search

Adds unit tests for the UnifiedSearcher and for UnifiedSearchBuilder.
These show up a problem with sorting (it doesn't filter out documents
for which the sort field doesn't exist).
Don't return documents missing sort field.

When sorting, we want to exclude documents which are missing the sort
field, since it doesn't really make sense to put those at either the top
or bottom of the search results.  This will support the ordering wanted
by https://www.pivotaltracker.com/story/show/66950384.