tests: energy_model: Test capacity margin

The capacity_margin_pct parameter of get_optimal_placement() can be set
to non-zero values to account for the overutilization band.

Add a self test with a task size larger than 80% of the CPU capacity
and a margin of 20% to ensure this results in an exception.

Suggested-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Quentin Perret <quentin.perret@arm.com>