Simplify PELT tests
- introduce an accurate PELT simulator in lisa/pelt.py
- Simplify the PELT tests by using a more accurate simulator:
- check that the behaviour of PELT is as expected = util matches the duty cycle of the periodic task
- check that PELT signal is correct = it closely matches the simulated one (average and maximum error of all samples).
Also fix an issue in TaskAnalysis.plot_task_activations().