tests/generic: Fix energy threshold for RampUp

The energy thresholds of several tests have been tweaked to account of
caveats of specific use-cases. In particular, the RampDown test has been
granted a 15% threshold to avoid too many issues with the capacity
margin. However, the RampUp test which has similar issues has been left
untouched.

Fix this.

Fixes: a31e7b99 ("tests/eas/generic: Tweak criteria for passing tests")
Signed-off-by: Quentin Perret <quentin.perret@arm.com>