tests/kernel/sched/schedule_api: Fix slice time test for fast ticks

When ticks are sub-millisecond, the math produces minimum and maximum
values for the slice duration test that are equal.  But because of
aliasing across tick boundaries, it's always possible (for any time
period, nothing specific to time slicing here) to measure one tick
more than an intended duration.  So make sure there's always at least
a range of 1ms.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>