[Tests] AsyncSubtensor (Part 6) (#2414)

* added test for `sign_and_send_extrinsic`

* added test for `get_children`

* added test for `get_subnet_hyperparameters`

* added test for `get_vote_data`

* added test for `is_hotkey_registered`

* added test for `get_uid_for_hotkey_on_subnet`

* added test for `blocks_since_last_update`

Author: basfroman