tests posix timer: Fix timer check

The timer check does not work properly for platforms
which have a system tick timer with a period that
does not properly divide the configured time interval.

In those, depending on when the timer is started and
stopped, one signal less may be received.

This is for example the case for nrf5x platforms,
where the system tick timer is driven by a 32.768KHz
clock.

=> Correct the test, to accept receiving 1 signal
too little during the wait.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>