Add unit test for tempest run --config-file

This commit adds a unit test for `tempest run --config-file`. That
if block was not tested in the gate. And an issue occurs like
this[0]. And this commit fixes a couple of issue in run.py through
adding this test.

[0] https://review.openstack.org/#/c/546822/

Change-Id: I235b822d802cf27d2dc43b35f85802d3a214b1b5
2 files changed