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

[delcypher]

Summary

ad890aa2327feb6b6aee676fe85b2352fba2403e 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

[delcypher]

@dvyukov Thanks for the review.

compiler-rt/test/tsan/flush_memory.cpp

-// RUN: %clangxx_tsan -O1 %s -o %t && %env_tsan_opts="flush_memory_ms=1 flush_symbolizer_ms=1 memory_limit_mb=1" %deflake %t | FileCheck %s
+// RUN: %clangxx_tsan -O1 %s -o %t
+// RUN: %env_tsan_opts="flush_memory_ms=1 flush_symbolizer_ms=1 memory_limit_mb=1" %deflake %run %t | FileCheck %s
 #include "test.h"
 
 long X, Y;
 
 void *Thread(void *arg) {
   __atomic_fetch_add(&X, 1, __ATOMIC_SEQ_CST);
   barrier_wait(&barrier);
   barrier_wait(&barrier);