tests/kernel/tickless/tickless_concept: Force 100 Hz ticks

This test was written to assume ~100 Hz ticks in ways that are
difficult to fix.  It wants to sleep for periods on the order of the
TICKLESS_IDLE_THRESH kconfig, which is extremely small on high tick
rate systems and (on nRF in particular) does not have a cleanly
divisible representation in milliseconds.

Fixing precision issues by cranking the idle threshold up on a
per-system basis seems like an abuse, as that is what we want to be
testing in the first place.  Just let the test run at the tick rate it
has always expected.

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