Be explicit about the DNS server used in tests.

transports/dns/src/lib.rs

@@ -591,19 +591,27 @@ mod tests {
 
         #[cfg(feature = "async-std")]
         {
+            // Be explicit about the resolver used. At least on github CI, TXT
+            // type record lookups may not work with the system DNS resolver.
+            let config = ResolverConfig::google();
+            let opts = ResolverOpts::default();
             async_std_crate::task::block_on(
-                DnsConfig::system(CustomTransport).then(|dns| run(dns.unwrap()))
+                DnsConfig::custom(CustomTransport, config, opts).then(|dns| run(dns.unwrap()))
             );
         }
 
         #[cfg(feature = "tokio")]
         {
+            // Be explicit about the resolver used. At least on github CI, TXT
+            // type record lookups may not work with the system DNS resolver.
+            let config = ResolverConfig::quad9();
+            let opts = ResolverOpts::default();
             let rt = tokio_crate::runtime::Builder::new_current_thread()
                 .enable_io()
                 .enable_time()
                 .build()
                 .unwrap();
-            rt.block_on(run(TokioDnsConfig::system(CustomTransport).unwrap()));
+            rt.block_on(run(TokioDnsConfig::custom(CustomTransport, config, opts).unwrap()));
         }
     }
 }