Use standard separator for TSan options in `stress.cpp` test case.

Use of space as a separator for options is problematic for wrapper
scripts (i.e. implementations of `%run`) that have to marshall
environment variables to target different than the host.

Rather than requiring every implementation of `%run` to support spaces
in `TSAN_OPTIONS` it is simpler to fix this single test case.

rdar://83637067

Differential Revision: https://reviews.llvm.org/D110967