test_futimens is flaky on ARM64 macOS.

on Apple Silicon macOS M1, the core test `test_futimens` (in both parameters `test_futimens_nodefs` and `test_futimens_rawfs`) fails occassionally, e.g.

[thinlto0.test_futimens_rawfs](http://clbri.com:8010/api/v2/logs/63940/raw_inline)
[llvmlibc.test_futimens_rawfs](http://clbri.com:8010/api/v2/logs/47305/raw_inline)
[corez.test_futimens_rawfs](http://clbri.com:8010/api/v2/logs/32155/raw_inline)
[thinlto1.test_futimens_nodefs](http://clbri.com:8010/api/v2/logs/29544/raw_inline)
[ltoz.test_futimens_nodefs](http://clbri.com:8010/api/v2/logs/29276/raw_inline)
[core2ss.test_futimens_nodefs](http://clbri.com:8010/api/v2/logs/27612/raw_inline)
[ltos.test_futimens_rawfs](http://clbri.com:8010/api/v2/logs/25910/raw_inline)

with a timestamp difference.

Reading the code, I am not quite able to find what could cause such a timestamp divergence. There does not seem to be room for such behavior as far as I could tell.

The test is not flaky on any other bots that I run (Windows or Linux). Specifically, it is not flaky on my macOS x64 bot either, only on macOS M1 bot.