Add support for new upstream-friendly load-tracking events

Created by: bjackman

There is a new upstream-friendly version 'sched_load_se' of the load-tracking trace events added by EAS. Add an abstraction so that LISA can work with both this new version and the older 'sched_load_avg_task' events.

Trace._sanitize_SchedLoadAvgTask is removed and the logic is moved into the new function _dfg_task_lt_events in tasks_analysis.

The users of sched_load_avg_cpu in tasks_analysis are updated to use the new _dfg method (except where their use is specific to the old sched_load_avg_cpu event).

Also add basic unit tests to exercise the code changed in this commit.