Adding unit tests for DIN70121 messages

[SebaLukas]

ISO Version

DIN70121

Describe the problem

It would be very useful for the library to have unit tests for each V2G message.

In the following table you will find the XML file and the corresponding EXI stream for each DIN message.

Message	Files	Done
SessionSetupReq	SessionSetupReq.zip	✔️
SessionSetupRes		❌
ServiceDiscoveryReq	ServiceDiscoveryReq.zip	✔️
ServiceDiscoveryRes	ServiceDiscoveryRes.zip	✔️
ServicePaymentSelectionReq	ServicePaymentSelectionReq.zip	❌
ServicePaymentSelectionRes	ServicePaymentSelectionRes.zip	❌
ContractAuthenticationReq	ContractAuthenticationReq.zip	❌
ContractAuthenticationRes	ContractAuthenticationRes.zip	❌
ChargeParameterDiscoveryReq	ChargeParameterDiscoveryReq.zip	❌
ChargeParameterDiscoveryRes	ChargeParameterDiscoveryRes.zip	❌
CableCheckReq	CableCheckReq.zip	❌
CableCheckRes	CableCheckRes.zip	❌
PreChargeReq	PreChargeReq.zip	❌
PreChargeRes	PreChargeRes.zip	❌
PowerDeliveryReq	PowerDeliveryReq.zip	❌
PowerDeliveryRes	PowerDeliveryRes.zip	❌
CurrentDemandReq	CurrentDemandReq.zip	❌
CurrentDemandRes	CurrentDemandRes.zip	❌
WeldingDetectionReq	WeldingDetectionReq.zip	❌
WeldingDetectionRes	WeldingDetectionRes.zip	❌
SessionStopReq	SessionStopReq.zip	❌
SessionStopRes	SessionStopRes.zip	❌

Describe your solution

Create for every message a encode and decode test with the help of XML and EXI messages.

A sample of how the tests can be written can be found in the tests/app_handshake folder.

Additional context