Add LB mu writer to decorate EventInfo in JetRecTestCfg

JetCalibTool now requires the EventInfo decorations, so it's necessary to add the LB mu writer at least to run on Run 2 AODs.

Might need some better testing to be future-proof, but this still seems to be under development.

@jbossios, be aware that it's now a bit of a pain to test JetCalibrationTool in master on older samples.

@chala, this seems to get the script running again.