public class X509Authentication : IClientAuthenticator
{
    protected readonly X509Certificate2 Certificate;

    public X509Authentication(X509Certificate2 certificate)
    {
        if (certificate == null) throw new ArgumentNullException("certificate");
        Certificate = certificate;
    }

    public HttpClient GenerateClient()
    {
        var clientHandler = new WebRequestHandler();
        clientHandler.ClientCertificates.Add(Certificate);
        var request = new HttpClient(clientHandler);
        return request;
    }
    public void Dispose()
    {
        //nothing to do here.
    }
}

[TestMethod]
public void HttpClientCreationIncludesCertificate()
{
    using (var auth = new X509Authentication(_certificate))
    using (var client = auth.GenerateClient())
    {
        Assert...what?  The certificate(s) are not visible here.
    }
}