tests. using the same port could make it fail on some build bots due to
failure in bind() with a conflicting socket in the time wait state.
this is a fragile workaround, but I expect we'll revist these test scenarios
with the introduction of socket creator anyway, so I chose to work this around
with a simplest fix.
namespace {
const char* const TEST_ADDRESS = "127.0.0.1";
-const char* const TEST_PORT = "53535";
+const char* const TEST_PORT = "53530";
// An internal exception class
class TestConfigError : public isc::Exception {