Add mq (message queue) to test infrastructure.

Add mtx (mutex) to test infrastructure, in order to avoid bootstrapping
complications that would result from directly using malloc_mutex.

Rename test infrastructure's thread abstraction from je_thread to thd.

Fix some header ordering issues.

test/include/test/thd.h

@@ -0,0 +1,9 @@
+/* Abstraction layer for threading in tests */
+#ifdef _WIN32
+typedef HANDLE thd_t;
+#else
+typedef pthread_t thd_t;
+#endif
+
+void	thd_create(thd_t *thd, void *(*proc)(void *), void *arg);
+void	thd_join(thd_t thd, void **ret);