tests/eas/generic: Tweak criteria for passing tests

- Relax energy threshold for ThreeSmallTasks because we don't really know what
  the ideal task placement is (don't expect energy model to be very accurate
  here), just want to sanity-check the placement (e.g. hopefully fail if tasks
  are all on big CPUs).

- Allow RampDown test a larger threshold to allow for hysteresis in
  down-migration.

- Remove the energy test from the TwoBigThreeSmall, this is similar to the
  ThreeSmall Tasks case but more extreme - let's just check that the throughput
  was provided but ignore the energy-aware placement as our estimation is very
  likely wrong for this workload.