tests: load_tracking: CPUMigrationBase: Use rtapp phase event
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.

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