tsan: deflake flush_memory.cpp

The test contains a race and checks that it's detected.
But the race may not be detected since we are doing aggressive flushes
and if the state flush happens between racing accesses, tsan won't
detect the race). So return 1 to make the test deterministic
regardless of the race.

Reviewed By: melver

Differential Revision: https://reviews.llvm.org/D115612