[TSan][Darwin] Deflake test

The asserted order of THREAD_DESTROY and end of main() is not
guaranteed:
```
 7: Hello from pthread
 8: THREAD_TERMINATE 0x7e8000104000, self: 0x7e8000104000, name: child thread
 9: Done.
10: THREAD_DESTROY 0x7e8000104000, self: 0x7e8000104000, name: child thread
```

Resulting in:
```
error: CHECK: expected string not found in input
// CHECK: Done.
```

Remove checking for "Done." (end of main()) to deflake this test.
Alternatively, we could use `CHECK-DAG`.

rdar://94036145