fix a heap_vector_map test under mac getdeps
fix a heap_vector_map test under mac getdeps

Summary:
Fixes a failing test observed in the Github Actions CI macOS build.

It is unclear what is the true root source of failure. There are multiple differences between build environments, and choice of standard library implementation is only one. But this tweak seems to work.

Reviewed By: Orvid

Differential Revision: D35905639

fbshipit-source-id: 64653cae376be19de95ab6fee97eb4e8a3f78dd3

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