Fix Conv.stdChronoToTimespec test on macOS
Fix Conv.stdChronoToTimespec test on macOS

Summary: [Folly] Fix `Conv.stdChronoToTimespec` test on macOS.

Reviewed By: simpkins

Differential Revision: D14575684

fbshipit-source-id: e0a0dfb89a17eaa5d83e1906f5d92112ab2b5b22

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly