Add BEAST_EXPECT macro:

This macro is used in the unit test framework to assist in
reporting the file and line number of test failures.

test/core/handler_alloc.cpp

@@ -18,7 +18,7 @@ class to_string_test : public beast::unit_test::suite
 public:
     void run()
     {
-        expect(to_string(boost::asio::const_buffers_1("x", 1)) == "x");
+        BEAST_EXPECT(to_string(boost::asio::const_buffers_1("x", 1)) == "x");
     }
 };