Continue name resolution fixing, add unit test

name_lookup() now returns all results given by getnameinfo(). When searching
for a server, finding one matching address in the lookup-results is enough for
a match.

Also, added a test for name_lookup(). The test is minimal on its own, as hardcoded
test cases are not generally valid.

server/modules/monitor/mariadbmon/mariadbmon.hh

@@ -172,12 +172,13 @@ private:
     class DNSResolver
     {
     public:
-        std::string resolve_server(const std::string& host);
+        using StringSet = std::unordered_set<std::string>;
+        StringSet resolve_server(const std::string& host);
 
     private:
         struct MapElement
         {
-            std::string    address;
+            StringSet   addresses; // A hostname can map to multiple addresses
             mxb::TimePoint timestamp;
         };