Are our many-seeds tests failing on Windows?

I just noticed this in our Windows CI logs:
```
  + ./miri many-seeds ./miri run tests/many-seeds/scoped-thread-leak.rs
  Trying seed: 0
  Failing seed: 0
```
There seem to be multiple things wrong here:
- Why is the test failing?
- Why is there no error message?
- Why does CI succeed anyway?