lisa.tests.scheduler.load_tracking: Unify PELTTask and Invariance
Unify Invariance and PELTTask tests, so that:
- duplicated behaviour check is removed (and a fair amount of helpers that come with that)
- PELT tests are carried on all types of CPUs, at multiple frequencies, giving much more coverage to the correctness test.
Note that this requires to have access to the PELT clock in the sched_load_se event, otherwise the simulated signal is completely wrong. The test is able to catch that though.
Note: Computations in PELT use an inaccurate delta of its clock, leading to some unwanted artifacts that happen to be reduced by other rounding errors. Since the simulator does not suffer from rounding errors, the artifacts become much more visible, which leads to a noticeable between the simulated signal and the one computed by the kernel.