Undo disabling MG C++ testing outputs for non-root processes

[seunghwak]

Disabling MG C++ testing outputs for non-root processes has an undesirable side effect of disabling error messages (e.g. exception outputs) as well. This makes failure diagnosis difficult in large-scale runs.

Delete the code disabling MG C++ testing outputs for non-root processes in this PR. In long run, we need a more elegant mechanism to disable only redundant test progress messages while still allowing error messages to be displayed.

[seunghwak]

Submitted #1616 for a longer term solution to disable only redundant test progress messages.

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (branch-21.06@570d811). Click here to learn what that means.
The diff coverage is n/a.

❗ Current head b4f3319 differs from pull request most recent head 17cc7a0. Consider uploading reports for the commit 17cc7a0 to get more accurate results

@@               Coverage Diff               @@
##             branch-21.06    #1615   +/-   ##
===============================================
  Coverage                ?   59.10%           
===============================================
  Files                   ?       79           
  Lines                   ?     3482           
  Branches                ?        0           
===============================================
  Hits                    ?     2058           
  Misses                  ?     1424           
  Partials                ?        0           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 570d811...17cc7a0. Read the comment docs.

[BradReesWork]

@gpucibot merge