fix Magic Folder tests

[daira]

daira: dawuud: there is a shallow reason why test_magicfolder_start_service doesn't work on the current 2489.write_downloaded_file.1 branch, and a deeper reason
daira: the shallow reason is that self.basedir needs to be created before self.local_dir
daira: the deeper reason is that create_invite_join_magic_folder depends on the client already existing
daira: so we cannot call it in the client_config_hook, because that's called in order to create the client
daira: and self.g.clients[0] is not set until it has returned (in fact, self.g is not set until all the config hooks have returned)
daira: so we do need to set up the grid first, then call create_invite_join_magic_folder, then restart the client
daira: I think the cleanest way to handle this is to add a restart_client(self, i=0) method to NoNetworkGrid
daira: (it would be asynchronous)
daira: incidentally, NoNetworkGrid is a misnomer because the client does listen on loopback to its web port. it's only the connections between clients and servers that are no-network
daira: (if the client didn't listen on its web port then create_invite_join_magic_folder wouldn't work)