Make async-io-test not fail on systems that don't support servname lookup.

It turns out this is more than just android. Some Debian Stretch docker image I'm working with seems to have this problem (but Stretch in general does not??).

c++/src/kj/async-io-test.c++

@@ -75,11 +75,12 @@ String tryParse(WaitScope& waitScope, Network& network, StringPtr text, uint por
   return network.parseAddress(text, portHint).wait(waitScope)->toString();
 }
 
-bool hasIpv6() {
-  // Can getaddrinfo() parse ipv6 addresses? This is only true if ipv6 is configured on at least
-  // one interface. (The loopback interface usually has it even if others don't... but not always.)
+bool systemSupportsAddress(StringPtr addr) {
+  // Can getaddrinfo() parse this addresses? This is only true if the address family (e.g., ipv6)
+  // is configured on at least one interface. (The loopback interface usually has both ipv4 and
+  // ipv6 configured, but not always.)
   struct addrinfo* list;
-  int status = getaddrinfo("::", nullptr, nullptr, &list);
+  int status = getaddrinfo(addr.cStr(), nullptr, nullptr, &list);
   if (status == 0) {
     freeaddrinfo(list);
     return true;
@@ -88,6 +89,7 @@ bool hasIpv6() {
   }
 }
 
+
 TEST(AsyncIo, AddressParsing) {
   auto ioContext = setupAsyncIo();
   auto& w = ioContext.waitScope;
@@ -103,20 +105,28 @@ TEST(AsyncIo, AddressParsing) {
 #endif
 
   // We can parse services by name...
-#if !__ANDROID__  // Service names not supported on Android for some reason?
-  EXPECT_EQ("1.2.3.4:80", tryParse(w, network, "1.2.3.4:http", 5678));
-  EXPECT_EQ("*:80", tryParse(w, network, "*:http", 5678));
-#endif
+  //
+  // For some reason, Android and some various Linux distros do not support service names.
+  if (systemSupportsAddress("1.2.3.4:http")) {
+    EXPECT_EQ("1.2.3.4:80", tryParse(w, network, "1.2.3.4:http", 5678));
+    EXPECT_EQ("*:80", tryParse(w, network, "*:http", 5678));
+  } else {
+    KJ_LOG(WARNING, "system does not support resolving service names on ipv4; skipping tests");
+  }
 
   // IPv6 tests. Annoyingly, these don't work on machines that don't have IPv6 configured on any
   // interfaces.
-  if (hasIpv6()) {
+  if (systemSupportsAddress("::")) {
     EXPECT_EQ("[::]:123", tryParse(w, network, "0::0", 123));
     EXPECT_EQ("[12ab:cd::34]:321", tryParse(w, network, "[12ab:cd:0::0:34]:321", 432));
-#if !__ANDROID__  // Service names not supported on Android for some reason?
-    EXPECT_EQ("[::]:80", tryParse(w, network, "[::]:http", 5678));
-    EXPECT_EQ("[12ab:cd::34]:80", tryParse(w, network, "[12ab:cd::34]:http", 5678));
-#endif
+    if (systemSupportsAddress("[12ab:cd::34]:http")) {
+      EXPECT_EQ("[::]:80", tryParse(w, network, "[::]:http", 5678));
+      EXPECT_EQ("[12ab:cd::34]:80", tryParse(w, network, "[12ab:cd::34]:http", 5678));
+    } else {
+      KJ_LOG(WARNING, "system does not support resolving service names on ipv6; skipping tests");
+    }
+  } else {
+    KJ_LOG(WARNING, "system does not support ipv6; skipping tests");
   }
 
   // It would be nice to test DNS lookup here but the test would not be very hermetic.  Even