Add disable_synonyms debug option

If debug=disable_synonyms is added to the query parameters, this
disables use of the synonym list by using an analyzer which doesn't
include synonyms in its list of actions.  This is useful for working out
whether synonyms are helping to fix results for particular queries, or
whether they're causing problems.
In test, consolidate query options into a method

This avoids repeating all the options being sent to a
UnifiedSearchBuilder in every test, and makes it easier to see which
ones are being set to special values for the purposes of the test.
Merge pull request #244 from alphagov/debug_disable_synonyms

Add disable_synonyms debug option