[BP] MB-18452: Single threaded test harness improvements

[BP] MB-18452: Single threaded test harness improvements

Refactor parts of the very new evp_store_single_threaded_test so that
it's simpler to drive tasks making new tests easier to write.

The main change is to provide helper methods for running any task from
a queue (with some checks) and a way to push a clean shutdown.

Change-Id: I7add574f0768c642f3c6c7c64293e882337a1cdc
Reviewed-on: http://review.couchbase.org/65928
Well-Formed: buildbot <build@couchbase.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: buildbot <build@couchbase.com>