ACL - Add CheckIP test(s)

Perfect candidate for a test using testcases/table driven (i.e. https://github.com/goftpd/goftpd/blob/master/acl/permissions_test.go#L21)

```
type test struct {
user string
raddr string
mask string
}
```

The test would have to make an ident listener on the laddr and respond to ident requests (can borrow an indetd server from here: https://git.sr.ht/~emersion/soju/tree/28cf1147e83c/ident.go#L47). Alternative would be to supply an ident check to the Authenticator as part of initialisation so that we don't have to do network integration tests and would simplify these tests significantly.