samples: canbus: isotp: add option for running in loopback mode

Add Kconfig option for running the CAN ISO-TP sample in loopback mode. This
removes the need for a fixture when running the sample via twister.

Exclude the loopback tests for kvaser,pcican. See
836f5826 for more information.

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>