Use id() for DummySigner comparison

Use id() for DummySigner comparison

If we give the `DummySigner` a valid identifier then we can use this to
do comparison.

Half the time we do comparison we only have a `dyn Signer` so we cannot
use `PartialEq`, add a helper function to check equality (this is in
test code so its not toooo ugly).

Thanks @afilini for the suggestion.