Enable bulking test on windows

[DickJC123]

Description

This PR is a response to #14368 and is now ready for merging. The first step here was to recreate the Windows CI failure, along with additional debugging output to show whether the intended bulking environment variable settings were making it to the point of use. I learned that the test's repeated setting and altering of environment variables was not getting through to the C++ runtime under Windows, which apparently gets a copy of the environment at process start-up. The fix was to spawn a new test process for each setting of different environment variables, as is already done for the gluon version of these same tests.

Thanks to @perdasilva for identifying this issue.

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

• The PR title starts with [MXNET-$JIRA_ID], where $JIRA_ID refers to the relevant JIRA issue created (except PRs with tiny changes)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage:
• Unit tests are added for small changes to verify correctness (e.g. adding a new operator)
• Nightly tests are added for complicated/long-running ones (e.g. changing distributed kvstore)
• Build tests will be added for build configuration changes (e.g. adding a new build option with NCCL)
• [X ] Code is well-documented:
• For user-facing API changes, API doc string has been updated.
• For new C++ functions in header files, their functionalities and arguments are documented.
• For new examples, README.md is added to explain the what the example does, the source of the dataset, expected performance on test set and reference to the original paper if applicable
• Check the API doc at http:https://mxnet-ci-doc.s3-accelerate.dualstack.amazonaws.com/PR-$PR_ID/$BUILD_ID/index.html
• [X ] To the my best knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change

Changes

• Feature1, tests, (and when applicable, API doc)
• Feature2, tests, (and when applicable, API doc)

Comments

• If this change is a backward incompatible change, why must this change be made.
• Interesting edge cases to note here

[karan6181]

@mxnet-label-bot add [CI, Test, Windows, pr-work-in-progress]]

[DickJC123]

If this latest commit passes CI, it will be ready for merging. The problem is that when one performs in Python:

os.environ[<key>] = <value>

this effects what the C++ backend sees only on Linux. Under Windows, some additional (and possibly windows-version-specific) work would be required to push the environment change to the msvcrt, which otherwise gets a copy of the environment at process start-up. My preferred fix here is to resort to the more bomb-proof approach of setting environment variables, then spawning a new process to perform the test (although admittedly this is bypassed on Python2 due to lack of spawn support).

[DickJC123]

Ready for your review. @perdasilva @eric-haibin-lin @szha