MB-9730 Add unit tests for `guide_purge_btree` API

Change-Id: I8377db68823f18cfb5d85190adbea17701337257
Reviewed-on: http://review.couchbase.org/30438
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>