AthenaMonitoringKernel: Change 2D histogram names to y_vs_x

Change the 2D histogram names from x_vs_y to the usual naming convention y_vs_x. The change is a trivial one line change in GenericMonitoringTool.py. The other changes in this MR are just to adapt the unit tests.

⚠ This change will impact any client that retrieves the histograms by names (e.g. monitoring displays, etc.). Therefore I would like to have explicit approval by DQ (@ponyisi). While I agree this change is potentially disruptive, now is still a good time to fix this before we have too many clients to adapt.

Thanks to @rbielski for pointing out this inconsistency.