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/websocket/detail/stream_base.cpp

@@ -21,7 +21,7 @@ class stream_base_test : public beast::unit_test::suite
 public:
     void testClamp()
     {
-        expect(detail::clamp(
+        BEAST_EXPECT(detail::clamp(
             std::numeric_limits<std::uint64_t>::max()) ==
                 std::numeric_limits<std::size_t>::max());
     }