wltests/wa_collector: add support for color bands to plot_comparisons

Created by: derkling

The plot_comparision is the most useful plot to get an overall view on what are the relative changes in power/performances among different kernels. However, to make more easy to have a fast check if a certain kernel is ok or not, it can be useful to define a fixes set of thresholds which allows to easily identify what's considered: ok, suspicious or definitively not good.

Let's add the possibility to define these three thresholds and use them to generate some color bands which allows to easily see where each configuration stands.

Signed-off-by: Patrick Bellasi patrick.bellasi@arm.com