core: better support for ad-hoc configs

properly reload/unload the relevant modules so hopefully no more weird hacks should be required

relevant
- https://github.com/karlicoss/promnesia/issues/340
- https://github.com/karlicoss/HPI/issues/46

tox.ini

@@ -20,9 +20,10 @@ passenv =
 commands =
     pip install -e .[testing]
     {envpython} -m pytest \
-                      tests/core.py      \
-                      tests/sqlite.py    \
-                      tests/get_files.py \
+                      tests/core.py            \
+                      tests/sqlite.py          \
+                      tests/get_files.py       \
+                      tests/test_tmp_config.py \
                       {posargs}