[ci] Default to n=2 for test parallelism (#12414)

* Revert "[skip ci] Revert "[ci] Default to n=2 for test parallelism (#12376)" (#12413)"

This reverts commit 478b672.

* [ci] Default to n=2 for test parallelism

This is attempt #2 of #12376 which was reverted in #12413. The changes
in `plugin.py` should keep all the tests on the same node so sporadic
failures don't happen due to scheduling.

Co-authored-by: driazati <driazati@users.noreply.github.com>

Author: driazati