lisa.tests: load_tracking: fix InvarianceItem workload
lisa.tests: load_tracking: fix InvarianceItem workload

Make sure the workload is small enough to execute at the wanted OPP. To achieve
that, aim at half the capacity of the OPP.

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