stestr run --pdb doesn't work for relative imports

[mtreinish]

Issue description

When running tests with the --pdb flag for tests in a directory that are relative imports are not working. For example if the tests are not included in the source of the project under test (ie a separate top level test/ directory). For example:

{0} unittest.loader._FailedTest.python [0.000093s] ... FAILED

Captured traceback:
~~~~~~~~~~~~~~~~~~~
    Traceback (most recent call last):
    ImportError: Failed to import test module: python
    Traceback (most recent call last):
      File "/usr/lib64/python3.7/unittest/loader.py", line 154, in loadTestsFromName
        module = __import__(module_name)
    ModuleNotFoundError: No module named 'test.python'
    
    

==============================
Failed 1 tests - output below:
==============================

unittest.loader._FailedTest.python
----------------------------------

Captured traceback:
~~~~~~~~~~~~~~~~~~~
    Traceback (most recent call last):
    ImportError: Failed to import test module: python
    Traceback (most recent call last):
      File "/usr/lib64/python3.7/unittest/loader.py", line 154, in loadTestsFromName
        module = __import__(module_name)
    ModuleNotFoundError: No module named 'test.python'
    

No discover, which uses a subprocess works fine. My guess is that the python interpreter doesn't have the test/ in it's pythonpath for whatever reason and can't find the module from the test id, but the source of the issue is not clear.

Expected behavior and actual behavior

The test case is run in the same process (ie pdb mode) instead of erroring.

Steps to reproduce the problem

Try to run any test case from https://github.com/Qiskit/qiskit-terra with the --pdb flag

Specifications like the version of the project, operating system, or hardware

System information

• OS: [e.g. Windows10, openSUSE 15.1, Ubuntu 18.04] Arch linux
• stestr version (stestr --version): 2.6.0 (and master)
• Python release (python --version): 3.7
• pip packages (pip freeze): N/A

Additional information