Add some tests

Test classes which I'm going to remove the MPL dependency from: blank,
allocator_utilities, is_xxx and reference_content.  I'm also going to change
indirect_traits, but that's tested in the python module.

test/blank_test.cpp

@@ -0,0 +1,32 @@
+
+// Copyright 2018 Daniel James.
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+#include <boost/blank.hpp>
+#include <boost/static_assert.hpp>
+#include <cassert>
+
+#if !defined(BOOST_NO_IOSTREAM)
+#include <sstream>
+#endif
+
+int main() {
+    BOOST_STATIC_ASSERT((boost::is_pod<boost::blank>::value));
+    BOOST_STATIC_ASSERT((boost::is_empty<boost::blank>::value));
+    BOOST_STATIC_ASSERT((boost::is_stateless<boost::blank>::value));
+
+    boost::blank b1,b2;
+    assert(b1 == b2);
+    assert(b1 <= b2);
+    assert(b1 >= b2);
+    assert(!(b1 != b2));
+    assert(!(b1 < b2));
+    assert(!(b1 > b2));
+
+#if !defined(BOOST_NO_IOSTREAM)
+    std::stringstream s;
+    s << "(" << b1 << ")";
+    assert(s.str() == "()");
+#endif
+}