samples: tracing: exclude mr_canhubk3 board

The mr_canhubk3 board enables by default an off-chip watchdog that must
be serviced to avoid triggering a reset and cannot be disabled on a
per-test basis.

The amount of data printed on this test prevents to initialize the
on-board watchdog within the expected window, causing a board reset.
Hence do not run the test on mr_canhubk3 board.

Signed-off-by: Manuel Argüelles <manuel.arguelles@nxp.com>