[ENH] Add tests for plot_series with pred_interval param

[aiwalter]

param pred_interval is not tested for the plot_series function, seems not to work as intended?

[MatthewMiddlehurst]

I wouldn't mind adding a test and checking to see if it's broken, but I'm not quite sure what the expected output is.

Do you have an example of what you want to happen?

[TonyBagnall]

I'll leave this issue open but I dont think we have enouigh information to call it a bug

[TonyBagnall]

I'm closing this as we do not let have forecasters with prediction intervals, or indeed any forecasters (see #2266). plot_series needs redesigning to take numpy arrays in any case