Test with IPv6 by default, and IPv4 only if that fails.

Change-Id: I1a17860245b7726a24576f5e1bddb0645171f28e
Reviewed-on: https://boringssl-review.googlesource.com/16486
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
diff --git a/ssl/test/runner/runner.go b/ssl/test/runner/runner.go
index 6239dba..10e728e 100644
--- a/ssl/test/runner/runner.go
+++ b/ssl/test/runner/runner.go
@@ -906,7 +906,10 @@
 		panic(fmt.Sprintf("The name of test %q suggests that it's version specific, but min/max version in the Config is %x/%x. One of them should probably be %x", test.name, test.config.MinVersion, test.config.MaxVersion, ver.version))
 	}
 
-	listener, err := net.ListenTCP("tcp4", &net.TCPAddr{IP: net.IP{127, 0, 0, 1}})
+	listener, err := net.ListenTCP("tcp", &net.TCPAddr{IP: net.IPv6loopback})
+	if err != nil {
+		listener, err = net.ListenTCP("tcp4", &net.TCPAddr{IP: net.IP{127, 0, 0, 1}})
+	}
 	if err != nil {
 		panic(err)
 	}