lisa: test: load_tracking: Add Invariance.test_(cpu|freq)_invariance

Add a "top-level" test that should fail if CPU or frequency invariance
is broken.

There is no foolproof way of doing that, since frequency invariance and
CPU invariance contributions cannot be observerd directly on the trace,
but that provides a heuristic comparable to the old CpuInvariance and
FrequencyInvariance tests.