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

ad890aa 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

compiler-rt/test/tsan/flush_memory.cpp

@@ -1,4 +1,5 @@
-// 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;