tests/eas/generic: Add plot_expected_util

Created by: valschneider

When running generics.py and trying to troubleshoot task_placement failures, it can sometime be difficult to understand what was the expected placement.

This commit adds a plot_expected_util method that will plot the per-CPU utilization that was expected. It is called by default during the test post-processing, and the plot will be saved as a .png file in the test results output directory.

An example of how this plot looks like can be found here: https://gist.github.com/valschneider/b8f15ce92c3581a0e7928ae80c8cf306