[tests] Get mailbox test running again

This commit fixes the mailbox tests in test_runner, which broke when
mailbox was simplified to just use call and respond. It also fixes a
bug the tests uncovered: if the mailbox is closed while a caller is in
the reply map (ie, when its call data has been passed on to a thread
calling respond, but has yet to be responded to itself), that caller is
never awoken.

src/kernel/objects/mailbox.cpp

@@ -26,6 +26,10 @@ mailbox::close()
 
     m_callers.clear(j6_status_closed);
     m_responders.clear(j6_status_closed);
+
+    util::scoped_lock lock {m_reply_lock};
+    for (auto &waiting : m_reply_map)
+        waiting.thread->wake(j6_status_closed);
 }
 
 j6_status_t