lisa.tests: load_tracking: Use LoadTrackingAnalysis.df_tasks_signal()
lisa.tests: load_tracking: Use LoadTrackingAnalysis.df_tasks_signal()

Avoid using df_tasks_signal*s*() since no new signals will be added to
it anymore (df_tasks_signal(signal) is the future-proof version).

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