tsan: don't use pipe2 in tests

MacOS buildbots failed:
stress.cpp:57:7: error: use of undeclared identifier 'pipe2'
https://green.lab.llvm.org/green//job/clang-stage1-RA/24209/consoleFull#-3468768778254eaf0-7326-4999-85b0-388101f2d404

Fix the test to not use pipe2.

Differential Revision: https://reviews.llvm.org/D110423