Add hash_is_avalanching_test.cpp

test/unordered/hash_is_avalanching_test.cpp

@@ -0,0 +1,25 @@
+// Copyright 2022 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/unordered/hash_traits.hpp>
+#include <boost/core/lightweight_test_trait.hpp>
+
+struct X1
+{
+};
+
+struct X2
+{
+    typedef void is_avalanching;
+};
+
+int main()
+{
+    using boost::unordered::hash_is_avalanching;
+
+    BOOST_TEST_TRAIT_FALSE((hash_is_avalanching<X1>));
+    BOOST_TEST_TRAIT_TRUE((hash_is_avalanching<X2>));
+
+    return boost::report_errors();
+}