CBQE-978: handle situation when test was not found in testrunner

for example, wrong path to test in conf file

Change-Id: I8e10d50585d5c1d73ead38feb22cc0d9cd9a358b
Reviewed-on: http://review.couchbase.org/23952
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>