Running tests through testrepository makes it difficult to use pdb for debugging them. oslotest includes oslo_debug_helper to make using pdb simpler/possible.
First, add a pdb call to the test code:
import pdb; pdb.set_trace()
Then run the tests through oslo_debug_helper like
$ oslo_debug_helper [tests to run]
or
$ tox -e venv -- oslo_debug_helper [tests to run]