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/mtx.h

@@ -0,0 +1,21 @@
+/*
+ * mtx is a slightly simplified version of malloc_mutex.  This code duplication
+ * is unfortunate, but there are allocator bootstrapping considerations that
+ * would leak into the test infrastructure if malloc_mutex were used directly
+ * in tests.
+ */
+
+typedef struct {
+#ifdef _WIN32
+	CRITICAL_SECTION	lock;
+#elif (defined(JEMALLOC_OSSPIN))
+	OSSpinLock		lock;
+#else
+	pthread_mutex_t		lock;
+#endif
+} mtx_t;
+
+bool	mtx_init(mtx_t *mtx);
+void	mtx_fini(mtx_t *mtx);
+void	mtx_lock(mtx_t *mtx);
+void	mtx_unlock(mtx_t *mtx);