[kernel] Replace endpoint with new mailbox API

The new mailbox kernel object API offers asynchronous message-based IPC
for sending data and handles between threads, as opposed to endpoint's
synchronous model.

src/user/test_runner/test_runner.module

@@ -10,6 +10,7 @@ module("test_runner",
 
         "tests/constexpr_hash.cpp",
         "tests/linked_list.cpp",
+        "tests/mailbox.cpp",
         "tests/map.cpp",
         "tests/vector.cpp",
     ])