Add explicit 200 test for dnslink-gateway

[lidel]

While refactoring #211 I notices we test _redirects on DNSLink gateway, but not really have a basic test for reading bytes when the Host points at DNSLink domain.

• confirm we relaly dont have it
• add it, or move to explicit file
• add proxy tests, similar to ones in tests/subdomain_gateway_proxy_test.go