lisa/tests/scheduler/load_tracking.py: Add Runqueue (root cfs_rq) Invariance tests
lisa/tests/scheduler/load_tracking.py: Add Runqueue (root cfs_rq) Invariance tests

Add class RqInvarianceItem(InvarianceItemBase) and class
RqInvariance(InvarianceBase). The runqueue invariance tests offer util
and load correctness tests.
InvarianceItemBase._get_trace_signal() gets an additional parameter
`cpus` so that it can be used for RqInvarianceItem._get_trace_signal()
too.

Signed-off-by: Dietmar Eggemann <b09ca079358248c3ff5bd91ea3781c6808cc6cf2@arm.com>

ARM-software/lisa,credp/lisa,ARM-software/lisa,ARM-software/lisa,credp/lisa,credp/lisa,ARM-software/lisa,credp/lisa