Make Test Helpers `no_std` Compatible (#823)

* Use `sp-application-crypto` when generating test justifications

* Try to justifications tests compiling

* Try using `ed25519_dalek` for justification creation

* Small cleanup

* Correctly generate a Keypair

The public key didn't correspond to what would be derived
with the given private key.

* Remove enum index hack

* Clean up test account helper functions

* Fix tests in `pallet-finality-verifier`

* Get `pallet-substrate-bridge` tests compiling again

* Use the correct keyring in justification tests

* Clean up Keyring related code a bit

* Appease Clippy

* Remove unused import

* Use keyring variants directly

* Remove unecessary From implementation