Add DNS upstream validation when it's being added

[ameshkov]

We'll be able to do it once this task in DnsLibs is done:
AdguardTeam/DnsLibs#31

Please note, that this is a long-running async operation:

1. You should specify a short timeout (2 or 3 seconds)
2. You should add some kind of a loader to the UI

[ameshkov]

Reopened, there's is no indication in the UI when the DNS server is being tested.