Magic Folder: add a restart_client method to NoNetworkGrid in order to fix 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)

[daira]

Work in progress branch ​https://github.com/tahoe-lafs/tahoe-lafs/2497.fix-magic-folder-tests.0.

[daira]

Fixed on the same branch.