Tidy up tests and docs:

* Add handler_ptr test and increase coverage
* Add test for prepare_buffer
* Move is_call_possible tests to a .cpp file
* Tidy up docs and declarations

test/core/handler_ptr.cpp

@@ -7,3 +7,74 @@
 
 // Test that header file is self-contained.
 #include <beast/core/handler_ptr.hpp>
+
+#include <beast/unit_test/suite.hpp>
+#include <exception>
+#include <utility>
+
+namespace beast {
+
+class handler_ptr_test : public beast::unit_test::suite
+{
+public:
+    struct handler
+    {
+        handler() = default;
+        handler(handler const&) = default;
+
+        void
+        operator()(bool& b) const
+        {
+            b = true;
+        }
+    };
+
+    struct T
+    {
+        T(handler&)
+        {
+        }
+
+        ~T()
+        {
+        }
+    };
+
+    struct U
+    {
+        U(handler&)
+        {
+            throw std::exception{};
+        }
+    };
+
+    void
+    run() override
+    {
+        handler h;
+        handler_ptr<T, handler> p1{h};
+        handler_ptr<T, handler> p2{p1};
+        try
+        {
+            handler_ptr<U, handler> p3{h};
+            fail();
+        }
+        catch(std::exception const&)
+        {
+            pass();
+        }
+        catch(...)
+        {
+            fail();
+        }
+        handler_ptr<T, handler> p4{std::move(h)};
+        bool b = false;
+        p4.invoke(std::ref(b));
+        BEAST_EXPECT(b);
+    }
+};
+
+BEAST_DEFINE_TESTSUITE(handler_ptr,core,beast);
+
+} // beast
+