tests/eas/generic.py: Tweak RampDown energy threshold

The RampDown test happens to fail on TC2 with a 16% or 17% energy delta,
while the behaviour is clearly correct. This happens because of the
overutilized band. The exact location of that band depends on the
capacity ratio of CPUs of the platform, but the RampDown test
configuration is hardcoded, without taking the platform's topology into
account. As a result, the test is hitting more sensible part of the
power-perf curve on some targets than others, and that is the cause of
the problems observed on TC2.

Fix this by tweaking the threshold at a level that should suit TC2
better. A proper fix would be to compute the threshold depending on the
EM of the target, but this is difficult to do in the current state of
the test framework. The new test framework should handle these scenarios
a lot better, so implementing 'the right thing' should be easier then.
In the meantime, the threshold tweak should workaround the issue just
fine.

Signed-off-by: Quentin Perret <quentin.perret@arm.com>