lisa.tests.staging.utilclamp: ignore preemption in task activation

Created by: vdonnefort

The test is skipping the first task activation, as the utilclamp values are set afterward. However, if this first activation is preempted, the preemption return would be seen as a new activation while it doesn't go through CPU selection, leading to a test failure.

Ignoring preemption fixes this issue and keeps the test sane.