x.py ignores config.toml's debug-assertions-std for unit tests

If you have in your config.toml file (or whichever file --config says), only this change to config.toml.example:

```toml
debug-assertions-std = true
```

debug assertions in src/liballoc or library/alloc used to be armed. Since #73374, debug assertions only go off if you also turn on `debug`. EDIT: since some point, even that doesn't help.

