Attempt to fix `nr-dlsim` Test 21 (4x4 MIMO, 2 Layers)

Nice catch by Francesco Mani

oai-nr-dlsim may seem fine but in fact, test21 may or may not be executed. On some run example, we do not have finished message on nr-dlsim pod.

I've put a fail criteria if we face time out.

The CI commit was merged.

But the fixes that Francesco made on the DL trigger an assertion on gNB in the NSA module.