[Support] Fix fragile ThreadPool test

The test ThreadPoolTest.AllThreads_UseAllRessources occasionally fails on the bots: http://lab.llvm.org:8011/builders/llvm-clang-lld-x86_64-scei-ps4-windows10pro-fast/builds/32015/steps/test-check-all/logs/FAIL%3A%20LLVM-Unit%3A%3AThreadPoolTest.AllThreads_UseAllRessources

This is because jobs were executed too fast on the first CPU socket, and never manage to reach the second CPU socket.

Differential Revision: https://reviews.llvm.org/D78832