regression-unit-test: wait for longer with a high workload

Change-Id: I7cd2f852e1e20130d68086b69fe7f9a9495e038d
Reviewed-on: http://review.couchbase.org/32981
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>