When running without all the subsystem events enabled in nOS-V, it is
possible to emit two consecutive VTx events, which would push twice the
same value ST_TASK_BODY into the subsystem channel. This change relaxes
the subsystem channel to accept duplicate stacked values. A regression
test is also added.
Reported-By: Raúl Peñacoba Veigas <raul.penacoba@bsc.es>
