asan: detect at-exit racy use-after-free more reliable

Today, asan may or may not report the following at-exit racy use-after-free.
To make the detection a bit more liable we may want to wait in the exit/_exit interceptor
for unfinished reports to complete.

```
#include
#include
int *a;
void Thread() {
usleep(1);
*a = 0;
}
int main() {
a = new int;
delete a;
std::thread t(Thread);
t.detach();
}
```

Start with the exit/_exit interceptor and trace how unfinished reports are handled during process termination. Run the provided detached-thread reproducer and compare repeated results; done means the at-exit racy use-after-free is detected more reliably after unfinished reports complete.

Written by the indexing model from the issue text.