Make postprocessing work with default ReFrame perflogs

[ilectra]

In general, the postprocessing doesn't depend on a specific format of the perflogs - at least that was the design plan. It does have some minimal requirements though, that should be inconsequential, but unfortunately they are not: they make it incompatible with the default perflogs produced by ReFrame (outside this framework) - as of version 4.5 at least. The reason is that among the few required columns is display_name, whereas the default perflogs contain short_name instead, which includes the same information.

It would be nice if our postprocessing would work seamlessly with default ReFrame perflogs, but it requires a bit more thinking than just changing the name of the required column, because ReFrame has a few variables to encode the benchmark name, and there is no guarantee which one(s) of those might stay in the default perflog in future versions.