Refactor extras:

A new directory extras/ is added, and code not part of the official
public Beast interface is moved there.

test/websocket/stream.cpp

@@ -17,7 +17,7 @@
 
 #include <beast/streambuf.hpp>
 #include <beast/to_string.hpp>
-#include <beast/detail/unit_test/suite.hpp>
+#include <beast/unit_test/suite.hpp>
 #include <boost/asio.hpp>
 #include <boost/asio/spawn.hpp>
 #include <boost/optional.hpp>
@@ -26,7 +26,7 @@ namespace beast {
 namespace websocket {
 
 class stream_test
-    : public beast::detail::unit_test::suite
+    : public beast::unit_test::suite
     , public test::enable_yield_to
 {
 public: