lisa.tests.scheduler.load_tracking: Create abstract InvarianceItemBase._get_trace_signal()
lisa.tests.scheduler.load_tracking: Create abstract InvarianceItemBase._get_trace_signal()

Allow that InvarianceItemBase.get_simulated_pelt() gets the correct
trace signal for class FooInvarianceItem(InvarianceItemBase) objects.
Each class FooInvarianceItem(InvarianceItemBase) has to overwrite
`_get_trace_signal()`.
Do this for class TaskInvarianceItem(InvarianceItemBase) here.

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

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