Update tox debug env

Having no path in debug environment assumes the test path to be
neutron_tempest_plugin/tests which doesn't exists, so tox -e debug
debug fails complaining that neutron_tempest_plugin/tests isn't
importable. This patch fixes the test path for debug environment.

Change-Id: Iac685d4d4fcee357c40141acc79b2aac28bd535b
1 file changed