Adapt `tsan/flush_memory.cpp` to run on non-local platforms.

ad890aa2 landed a test without
using the `%run` prefix which means the test fails to run for
platforms that need it (e.g. iOS simulators).

This patch adds the `%run` prefix. While we're here also split
the single `RUN` line into two to make debugging easier.

rdar://83637296

Differential Revision: https://reviews.llvm.org/D110734