A to-be-added UE's RA is added through nr_mac_add_test_ue() (the name is
misleading, it is not a "test UE") through its CellGroupConfig. Using
the process_CellGroupConfig() is unfortunate, because it is not really
clear what part of the CellGroupConfig gets evaluated (it is certainly
not all). Move the code for adding a new RA directly in the user-add
function, and refactor this one.