public async Task TestSignature_UserPassword()
{
var firstBeleg = new MachineReadableCode(TrustProvider.A_Trust, "BOX1", "1", DateTime.UtcNow, 0m, 0m, 0m, 0m, 0m, "00==", _certHexSerial, "prev===");
var signature = await _client.Sign(firstBeleg, ICredentials.CreateByUsernamePassword("u123456789", "123456789"));
signature.IsSuccessful.Should().BeTrue(because: signature.ErrorMessage);
signature.Payload.Should().NotBeNull();
signature.Payload.Signature.Should().NotBeNullOrEmpty();
}