Right now set_mode() on GprsMs behaves in pretty counter-intuitive way:
* it's possible to set current DL MCS higher than max value
* EGPRS and EGPRS_GMSK have the same max DL MCS
* setting EGPRS* mode drops current/max MCS values to unknown
Let's capture this in a unit-test before attempting any further
modifications.
Change-Id: Ibf917f4b49d927a21cbd467775806fa6ea06a6a6