Implement feature test for TXT Buffer states.

E.g. insert frame, transmit X times, verify that buffer ends in OK. Abort frame and verify that buffer ends in aborted. Force errors and verify buffer ends up in Error state.

Try to intensively "set_ready", "set_abort" and see if the whole TX chain (TX Buffer, TX Arbitrator and CAN Core) are consistent with SW, HW commands.

Try to insert multiple frames and check if truly the lowest priority one is selected.