[Sanitizer] Deflake TSan test (#61166)

This test does:
```
race()
print("Done!")

// CHECK: ThreadSanitizer: data race
// CHECK: Done!
```

We see some recent cases where the output of the test binary on iOS
devices was:
```
Done!
==================
WARNING: ThreadSanitizer: data race
…
```

So apparently the TSan report output is not guaranteed to be printed
before "Done!".  Maybe this is because we print "Done!" on stdout and
the sanitizer report on stderr?

The remaining question is: what changed that we are seeing this issue
now, but not previously?

rdar://99713724

Co-authored-by: Julian Lettner <[email protected]>

Author: yln

test/Sanitizers/tsan/libdispatch.swift

@@ -49,5 +49,5 @@ for _ in 1...10 {
 
 print("Done!")
 
-// CHECK: ThreadSanitizer: data race
-// CHECK: Done!
+// CHECK-DAG: ThreadSanitizer: data race
+// CHECK-DAG: Done!