Some tests location were missing in the `TEST_DIRS` make variable.
Also add a commit to remove explicit `pytest_plugins` declaration.
This makes pytest fails in a virtualenv (see below) as it attempts to load the `aiohttp`
pytest plugin multiple times so prefer to rely on `pytest-aiohttp` instead.
```
12:50 $ pytest -sv swh/core/api/tests/
============================================================================================== test session starts ===============================================================================================
platform linux -- Python 3.7.3, pytest-6.1.2, py-1.9.0, pluggy-0.13.1 -- /home/anlambert/.virtualenvs/swh/bin/python3
cachedir: .pytest_cache
hypothesis profile 'default' -> database=DirectoryBasedExampleDatabase('/home/anlambert/swh/swh-environment/swh-core/.hypothesis/examples')
rootdir: /home/anlambert/swh/swh-environment/swh-core, configfile: pytest.ini
plugins: aiohttp-0.3.0, asyncio-0.14.0, django-3.9.0, mock-3.3.1, requests-mock-1.8.0, hypothesis-5.33.1, flask-1.0.0, forked-1.3.0, xdist-2.1.0, dash-1.16.0, postgresql-2.4.1, swh.journal-0.4.4.dev1+g7815cad, swh.core-0.8.1.dev4+g39d8579.d20201119
collected 46 items / 1 error / 45 selected
===================================================================================================== ERRORS =====================================================================================================
_______________________________________________________________________________ ERROR collecting swh/core/api/tests/test_async.py ________________________________________________________________________________
../../../.virtualenvs/swh/lib/python3.7/site-packages/pluggy/manager.py:127: in register
hook._maybe_apply_history(hookimpl)
../../../.virtualenvs/swh/lib/python3.7/site-packages/pluggy/hooks.py:333: in _maybe_apply_history
res = self._hookexec(self, [method], kwargs)
../../../.virtualenvs/swh/lib/python3.7/site-packages/pluggy/manager.py:93: in _hookexec
return self._inner_hookexec(hook, methods, kwargs)
../../../.virtualenvs/swh/lib/python3.7/site-packages/pluggy/manager.py:87: in <lambda>
firstresult=hook.spec.opts.get("firstresult") if hook.spec else False,
../../../.virtualenvs/swh/lib/python3.7/site-packages/aiohttp/pytest_plugin.py:38: in pytest_addoption
help="run tests faster by disabling extra checks",
../../../.virtualenvs/swh/lib/python3.7/site-packages/_pytest/config/argparsing.py:96: in addoption
self._anonymous.addoption(*opts, **attrs)
../../../.virtualenvs/swh/lib/python3.7/site-packages/_pytest/config/argparsing.py:352: in addoption
raise ValueError("option names %s already added" % conflict)
E ValueError: option names {'--aiohttp-fast'} already added
```