mpi-example-nms-2 test is failing

commit 7be87100 on handling multiple wifi connections with same channel object is causing one of the MPI tests (that makes use of NixVector) to fail.

With MPI enabled, this command:

$ ./test.py -s mpi-example-nms-2 -t out

yields this output:

FAIL: Test Suite "mpi-example-nms-2" (2927.380)
FAIL: Test Case "mpi-example-nms-2" (2927.380)
    Details:
      Message:   ASCII traces testpy-output/2021-08-28-23-29-37-CUT/mpi-example-nms-2/mpi-example-nms-2/mpi-example-nms-2.reflog and /home/buildslave/jenkins/workspace/valgrind/label/Ubuntu-21.04/ns-3-allinone/ns-3-dev/src/mpi/test/mpi-example-nms-2.reflog differ starting from line 19
      Condition: diff (actual) == false (limit)
      Actual:    1
      Limit:     0
      File:      ../src/core/model/example-as-test.cc
      Line:      127

If we look at the difference, it is that;

TEST : 00018 : FAILED  Observed sink traces (1400) not equal to expected (2400)
TEST : 00019 : -----

I don't know anything about this test/example but it is a large network simulation. It may be the case that the previous behavior was incorrect and the correct number of sink traces should be 1400 and not 2400, but this needs to be checked (either fix the code or fix the expected number of sink traces).