tests: added test for mutex unlock order

tests/mutex_order/Makefile

+APPLICATION = mutex_order
+include ../Makefile.tests_common
+
+BOARD_INSUFFICIENT_MEMORY := stm32f0discovery weio
+
+USEMODULE += xtimer
+
+include $(RIOTBASE)/Makefile.include

tests/mutex_order/README.md

+Expected result
+===============
+When successful, you should see 10 different threads printing their PID and
+priority. The thread with the lowest priority will print its status first,
+followed by the other threads in the order of their priority (highest next). The
+output should look like the following:
+
+```
+main(): This is RIOT! (Version: xxx)
+Mutex order test
+Please refer to the README.md for more information
+
+T3 (prio 6): locking mutex now
+T4 (prio 0): locking mutex now
+T5 (prio 4): locking mutex now
+T6 (prio 2): locking mutex now
+T7 (prio 1): locking mutex now
+T3 (prio 6): unlocking mutex now
+T4 (prio 0): unlocking mutex now
+T7 (prio 1): unlocking mutex now
+T6 (prio 2): unlocking mutex now
+T5 (prio 4): unlocking mutex now
+
+Test END, check the order of priorities above.
+```
+
+Background
+==========
+This test application stresses a mutex with a number of threads waiting on it.

tests/mutex_order/main.c

+/*
+ * Copyright (C) 2016 Freie Universität Berlin
+ *
+ * This file is subject to the terms and conditions of the GNU Lesser
+ * General Public License v2.1. See the file LICENSE in the top level
+ * directory for more details.
+ */
+
+/**
+ * @ingroup     tests
+ * @{
+ *
+ * @file
+ * @brief       Test application for testing mutexes
+ *
+ * @author      Hauke Petersen <hauke.petersen@fu-berlin.de>
+ * @}
+ */
+
+#include <stdio.h>
+
+#include "mutex.h"
+#include "thread.h"
+#include "xtimer.h"
+
+#define THREAD_NUMOF            (5U)
+#define DELAY                   (10 * 1000U)        /* 10ms */
+
+extern volatile thread_t *sched_active_thread;
+
+static char stacks[THREAD_NUMOF][THREAD_STACKSIZE_MAIN];
+
+static const char prios[THREAD_NUMOF] = {THREAD_PRIORITY_MAIN - 1, 4, 0, 2, 1};
+
+static mutex_t testlock;
+
+static void *lockme(void *arg)
+{
+    (void)arg;
+    volatile thread_t *t = sched_active_thread;
+
+    printf("T%i (prio %i): locking mutex now\n",
+           (int)t->pid, (int)t->priority);
+    mutex_lock(&testlock);
+
+    xtimer_usleep(DELAY);
+
+    printf("T%i (prio %i): unlocking mutex now\n",
+           (int)t->pid, (int)t->priority);
+    mutex_unlock(&testlock);
+
+    return NULL;
+}
+
+int main(void)
+{
+    puts("Mutex order test");
+    puts("Please refer to the README.md for more information\n");
+
+    mutex_init(&testlock);
+
+    /* create threads */
+    for (unsigned i = 0; i < THREAD_NUMOF; i++) {
+        thread_create(stacks[i], sizeof(stacks[i]), prios[i], 0,
+                      lockme, NULL, "t");
+    }
+
+    mutex_lock(&testlock);
+    puts("\nTest END, check the order of priorities above.");
+
+    return 0;
+}