Use correct base test class for test_stress unit tests

This commit switches the stress test unit tests to use the correct
base test class. It was previously using the tempest test base test
class instead of the unit test one.

Change-Id: I518d5b50abbd8c54335e7d5f1a53e0c1b6e36673
Merge branch 'master' of https://github.com/openstack/tempest