tests: load_tracking: CPUMigrationBase: Use rtapp phase event
Accurately find the start and end of each rtapp phase using the events from rtapp, rather than blindly relying on the rtapp profile.
Depends on https://github.com/ARM-software/lisa/pull/1266 since signal convergence is fully delegated to the buffer phase, which means it has to run on the right CPUs to allow run-queue signals to converge.