Allow test::stream to be used with NORTTI:

Currently, the test stream code doesn't compile when both debug builds
(namely assertions) are enabled along with disabling rtti.  The current
code directly uses type_id(), which isn't available without rtti.

This commit allows stream.hpp to compile without rtti enabled.

Signed-off-by: Ed Tanous <ed@tanous.net>

close #2331

include/boost/beast/_experimental/test/stream.hpp

@@ -214,7 +214,7 @@ public:
     : in_(std::move(other.in_))
     , out_(std::move(other.out_))
     {
-        assert(in_->exec.target_type() == typeid(Executor2));
+        BOOST_ASSERT(in_->exec.template target<Executor2>() != nullptr);
         in_->exec = executor_type(*in_->exec.template target<Executor2>());
     }