'Gets a list of fixtures which are applicable to the given node id.
:param str argname: name of the fixture to search for
:param str nodeid: full node id of the requesting test.
:return: list[FixtureDef]'

def getfixturedefs(self, argname, nodeid):

    try:
        fixturedefs = self._arg2fixturedefs[argname]
    except KeyError:
        return None
    else:
        return tuple(self._matchfactories(fixturedefs, nodeid))
