Larger Test Keyring Support (#842)

* Allow creation of authority lists with any number of authorities

* Move keyring helpers into their own module

* Add helper for generating list of test accounts

* Fix import names in tests

* Rename Keyring trait to Signer

* Get list of accounts in a more functional way

* Clarify meaning of `test_keyring` return type

* Use concrete test account type instead of generics

* Make sure voter set contains all authorities which signed off on pre-commits