set -e when checking testr test counts.

* modules/jenkins/files/slave_scripts/run-tox.sh: set -e when checking
test counts so that the case when no tests are found and test complains
about a missing integer expression we fail the Jenkins job.

Change-Id: I2b416637d02d0b87fd3f9ece323e168c039da38b
Reviewed-on: https://review.openstack.org/30818
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Tested-by: Jenkins