CBD-1249:  call 'testrunner', not 'testrunner.py'

Change-Id: Ib84f2b927143dfc48eaa24f5f0a8b5c581719e6b
Reviewed-on: http://review.couchbase.org/39063
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>