UE MAC: in phy-test/do-ra, fill TB randomly

Commit d641a761 ("fix BSR report malformed, add SHORT BSR when it can") removed code to fill the remainder of a TB, if it is not filled with data from RLC, with random data. This random data filling is intentional, as it might prove useful for demos or when debugging (e.g., to see used spectrum instead of 0, etc.). Reintroduce the functionality, but unlike the original code, make it dependent on the mode, not RFsim.

See also discussion in #888 (closed). The same is done in the gNB, in gNB_scheduler_dlsch.c.

Fixup: d641a761