Checking existence of LastTest.log

Change-Id: I0c8a6ded37d0de964da78742005ed2195b7f3da1
Reviewed-on: http://review.couchbase.org/38749
Reviewed-by: Eric Limare <eric.limare@couchbase.com>
Tested-by: Eric Limare <eric.limare@couchbase.com>