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

This reverts commit 369e8b2.

There are certain tests that need to be serialized first before this can
merge or else failures like
https://ci.tlcpack.ai/job/tvm/job/main/4040/display/redirect will happen
based on which tests happen to be run together or not

Author: driazati