Provide a tool/option to verify certificates

[rnhmjoj]

When implementing key-rotation the following command was used to check the validity of the certificate/ephemeral keys.

dnscrypt-proxy --resolver-address=127.0.0.1:443 \
                 --provider-name=2.dnscrypt-cert.<yourdomain> \
                 --provider-key=<provider_public_key> \
                 --test=10080

In dnscrypt-proxy 2 --test is gone, see DNSCrypt/dnscrypt-proxy#185 and there is no other way than using the old v1 fork.

This mode should be provided by a small tool or, ideally, by dnscrypt-wrapper itself.