[Request] Add Test button

I find it hard to read, if Simple DNSCrpyt does it job at all and besides an [animated tray icon](https://github.com/bitbeans/SimpleDnsCrypt/issues/315), it might be nie to have a more telling "Test" function included:

https://www.dnsleaktest.com/results.html - DNS server IP Address
https://whoer.net/ru - DNS Resolver IP Address/Country, etcetera...
http://entropy.dns-oarc.net/test DNS Entrophy
http://en.conn.internet.nl/connection - DNSSEC
https://rootcanary.org/test.html - DNS Algorithm

_(I have cp the list above from https://github.com/bitbeans/SimpleDnsCrypt/issues/327)_