refactor: refactor DNS resolution

This commit also adds some of tests for the Domain model. It was during the writing of these tests that the DNS resolution refactoring requirement became apparent.

app/models/track_domain.rb

@@ -54,8 +54,7 @@ class TrackDomain < ApplicationRecord
   end
 
   def check_dns
-    result = domain.resolver.getresources(full_name, Resolv::DNS::Resource::IN::CNAME)
-    records = result.map { |r| r.name.to_s.downcase }
+    records = domain.resolver.cname(full_name)
     if records.empty?
       self.dns_status = "Missing"
       self.dns_error = "There is no record at #{full_name}"