lisa.tests.scheduler.load_tracking: Add assert to check duty_cycle_pct > 0

Occasionally the load_tracking invariance tests
[load\|util]_[correctness\|behaviour] fail on rb5 little/medium CPUs
on low frequencies.
The root cause is wrong rt-app calibration for these CPUs/frequencies.
The reason for the rt-app calibration issue is not known at the moment.
Add an assert in _get_rtapp_profile() of class InvarianceItemBase to
catch the issue earlier.

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>