pyagl J1939 CAN tests need to be marked xfail

ATM the pyagl tests for J1939 support in the CAN binding fail in CIAT in the instance of changes to the agl-service-can-low-level repo being tested with the standalone CI build with the SDK.  This is because J1939 support is only being enabled for the qemu platforms via an override in the binding recipe, yielding different results for widget vs image CI tests.  To work around this for now, it has been decided to mark the tests as expected fail (xfail).  A separate issue will be opened to track follow up work to potentially enable J1939 support in the binding by default.