My unit tests can be started individually from the context menu. I can also run all of them using something like
nosetests ./*tests.py
from within that specific folder containing the tests.
I played around a while with PyCharm to find a run configuration that actually allows me to runn all tests from all files within a folder at mouseclick.
Create a run configuration using Edit Configurations from the menu, add one with the ‚+‘ sign and choose any python unittest that suits your needs from the list of options:

The tricky bit is the pattern. My test unit files all end in *tests.py – I played aroung ab bit, until the regex really worked:
^.*tests.py$ .*tests.py$ .*tests.py
I settled for the middle option, but all 3 work.