tests: skip IPv6 tests if IPv6 is not available

This commit also renames the getAvailableInterfaceIp() API
to getTestIp(), and improves its expressiveness.

Change-Id: I4b0080fb5e85cd523305ecb14183ef6acb34b5cd
Refs: #3367
diff --git a/tests/daemon/face/tcp-transport-fixture.hpp b/tests/daemon/face/tcp-transport-fixture.hpp
index e68eff9..97b195f 100644
--- a/tests/daemon/face/tcp-transport-fixture.hpp
+++ b/tests/daemon/face/tcp-transport-fixture.hpp
@@ -30,6 +30,7 @@
 #include "face/face.hpp"
 
 #include "dummy-receive-link-service.hpp"
+#include "test-ip.hpp"
 #include "tests/limited-io.hpp"
 
 namespace nfd {