RNTupleART: Add a new test for reading/writing ## containers in RNTuple
Adding a new ART test that runs a full-chain MC job that writes ##
containers in RNTuple
in addition to event data. The test is currently set to run in main--dev3LCG
nightly because accessing strings in RNTuple
on the python side doesn't work except against the head of ROOT master.
cc: @gemmeren