unittest is not able to discover / run tests

There are some related questions, but none apply.

This is my directory tree:

» tree abc_backend
abc_backend/
├── backend_main.py
├── FundDatabase.db
├── healthcheck.py
├── __init__.py
├── init.py
├── portfolio.py
├── private.py
├── __pycache__
├── questionnaire.py
├── recurring.py
├── registration.py
├── tests
│   ├── config.py
│   ├── __init__.py
│   ├── __pycache__
│   ├── test_backend.py
│   ├── test_healthcheck.py
│   └── test_private.py
├── trading.py
├── Users.db
├── VERSION
└── visualisation.py

unittest is not able to find anything:

top » python -m unittest abc_backend

----------------------------------------------------------------------
Ran 0 tests in 0.000s

OK

Not even from within abc_backend:

abc_backend » python -m unittest tests

----------------------------------------------------------------------
Ran 0 tests in 0.000s

OK

What I have already verified:

• my test methods are properly named (test_whatever)
• my testcases extend unittest.TestCase
• the abc_backend and the abc_backend/tests directories have an (empty) __init__.py
• all test modules are importable (see below)
• unittest discover finds the tests, but has problems with relative imports (see below)
• nose is able to discover and run the tests, no problems

I would like to understand:

• why do I need to pass discover to unittest to force it to discover the tests? What does unittest do without the discover sub-command? (I thought unittest does test discovery by default). According to the documentation:

python -m unittest is the equivalent of python -m unittest discover

• once the tests are discovered (by forcing the discover sub-command), why do I have import issues?

Test modules are importable

» python
Python 3.4.3 (default, Oct 14 2015, 20:28:29) 
[GCC 4.8.4] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import abc_backend.tests
>>> import abc_backend.tests.test_private
>>> import abc_backend.tests.test_healthcheck
>>> import abc_backend.tests.test_backend

unittest discover has problems with relative imports

If I run it from the top dir:

top » python -m unittest discover abc_backend
======================================================================
ERROR: tests.test_private (unittest.loader.ModuleImportFailure)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/lib/python3.4/unittest/case.py", line 58, in testPartExecutor
    yield
  File "/usr/lib/python3.4/unittest/case.py", line 577, in run
    testMethod()
  File "/usr/lib/python3.4/unittest/loader.py", line 32, in testFailure
    raise exception
ImportError: Failed to import test module: tests.test_private
Traceback (most recent call last):
  File "/usr/lib/python3.4/unittest/loader.py", line 312, in _find_tests
    module = self._get_module_from_name(name)
  File "/usr/lib/python3.4/unittest/loader.py", line 290, in _get_module_from_name
    __import__(name)
  File "/foo/bar/abc_backend/tests/test_private.py", line 6, in <module>
    from .. import init
ValueError: attempted relative import beyond top-level package

If I run it from within abc_backend:

abc_backend » python -m unittest discover tests

======================================================================
ERROR: test_private (unittest.loader.ModuleImportFailure)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/lib/python3.4/unittest/case.py", line 58, in testPartExecutor
    yield
  File "/usr/lib/python3.4/unittest/case.py", line 577, in run
    testMethod()
  File "/usr/lib/python3.4/unittest/loader.py", line 32, in testFailure
    raise exception
ImportError: Failed to import test module: test_private
Traceback (most recent call last):
  File "/usr/lib/python3.4/unittest/loader.py", line 312, in _find_tests
    module = self._get_module_from_name(name)
  File "/usr/lib/python3.4/unittest/loader.py", line 290, in _get_module_from_name
    __import__(name)
  File "/foo/bar/abc_backend/tests/test_private.py", line 6, in <module>
    from .. import init
SystemError: Parent module '' not loaded, cannot perform relative import