tsan: switch sync test from EXPECT to CHECK

Gtest's EXPECT calls whole lot of libc functions
(mem*, malloc) even when EXPECT does not fail.
This does not play well with tsan runtime unit tests
b/c e.g. we call some EXPECTs with runtime mutexes locked.

Reviewed By: melver

Differential Revision: https://reviews.llvm.org/D112601