acceptance/csharp: the dotnet pg connector expects a db named after the username

Pursuant to the changes in #22371, the csharp test fails with:

```
Unhandled Exception: Npgsql.PostgresException: 3D000: database "root" does not exist
```

I surmise that it wants a database named after the username to pre-exist. Not sure how to override.

Might choose to skip until resolution is found.