[bugfix] Fix relative imports in deeply nested tests

[vkarak]

This is practically a follow up of #3110 by fixing relative imports of deeply nested tests. Currently, given the following structure:

unittests/resources/checks_unlisted/testlib/
├── __init__.py
├── nested
│   ├── __init__.py
│   └── dummy.py
├── simple.py
└── utility
    └── __init__.py

the following import in nested/dummy.py will crash:

from ..utility import dummy_fixture