dnsdist: Increase the time granted for the "timeout then restart" test

dnsdist: Increase the time granted for the "timeout then restart" test

This test fails from time to time in our CI. After investigation, it
turns out that sometimes the response comes just a few milliseconds
after the 3 seconds timeout has expired:

```
Got answer from 127.0.0.1:14191, relayed to 127.0.0.1:36139 (UDP), took 3.00105e+06 us
```

So let's use a slightly larger timeout of 4 seconds. It will not add
any delay if the response is received faster than that and will hopefully
reduce the number of spurious failures.

Signed-off-by: Remi Gacogne <remi.gacogne@powerdns.com>