test(protocol): add coverage to 256bit ecdsa curve

[ArnaudBrousseau]

[this repost of https://github.com/duo-labs/webauthn/pull/158, after seeing https://github.com/duo-labs/webauthn/issues/155]

For a few hours I was convinced that this library (webauthncose package specifically) wasn't doing its job properly because my code was accepting bad signatures as valid. So I set out to write a test to capture this. Turns out: this library is doing its job, and my code was at fault!

Shameful details

func checkWebauthnSignature(...) error {
    valid, err := webauthncose.VerifySignature(key, message, sigBytes)
    if !valid || err != nil {
        return errors.Wrap(err, "error verifying signature")
    }
}

The bug in the above snippet: errors.Wrap(nil, "something") is nil. Which means when valid is false, nil is returned 🤦

Instead of discarding this test I thought it'd be a good addition to the test suite.

[james-d-elliott]

LGTM