Add stress test for heavy batch free

[mjp41]

@wareya provided an interesting benchmark to mimalloc. microsoft/mimalloc#1002 (comment)

@wareya, I would like to add a modified version to stress snmalloc's backend.

[wareya]

Thumbs up, and you have permission to relicense it however you need to (I have a copy on one of my repositories under CC0).

[mjp41]

Thanks, I'll add you as a Coauthor to the Commit, so you should appear in the release notes for the next release.

[mjp41]

@wareya thanks for the example, good luck with wmalloc.