sys/test_utils/dummy_thread: initial commit

This commit adds a module that just creates a thread that does nothing,
at boot time, in auto_init().

sys/Makefile

@@ -31,6 +31,9 @@ endif
 ifneq (,$(filter test_utils_interactive_sync,$(USEMODULE)))
   DIRS += test_utils/interactive_sync
 endif
+ifneq (,$(filter dummy_thread,$(USEMODULE)))
+  DIRS += test_utils/dummy_thread
+endif
 ifneq (,$(filter net_help,$(USEMODULE)))
   DIRS += net/crosslayer/net_help
 endif

sys/auto_init/auto_init.c

@@ -49,6 +49,10 @@ void auto_init(void)
         extern void init_schedstatistics(void);
         init_schedstatistics();
     }
+    if (IS_USED(MODULE_DUMMY_THREAD)) {
+        extern void dummy_thread_create(void);
+        dummy_thread_create();
+    }
     if (IS_USED(MODULE_EVENT_THREAD)) {
         LOG_DEBUG("Auto init event threads.\n");
         extern void auto_init_event_thread(void);

sys/test_utils/dummy_thread/Makefile

@@ -0,0 +1 @@
+include $(RIOTBASE)/Makefile.base

sys/test_utils/dummy_thread/dummy_thread.c

@@ -0,0 +1,57 @@
+/*
+ * Copyright (C) 2020 Kaspar Schleiser <kaspar@schleiser.de>
+ *                    Freie Universität Berlin
+ *                    Inria
+ *
+ * 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     sys
+ * @{
+ *
+ * @file
+ * @brief       Module creating a dummy thread
+ *
+ * This module can be used to mess up the number of threads a bit, e.g., for
+ * testing test scripts.
+ *
+ * This module can be used by manually adding it to the command line when
+ * building, e.g.,
+ *
+ *     USEMODULE+=dummy_thread make -C tests/rmutex
+ *     make -C tests/rmutex test
+ *
+ * Note how the output of the test changes compared to a build without
+ * dummy_thread.
+ *
+ * @author      Kaspar Schleiser <kaspar@schleiser.de>
+ *
+ * @}
+ */
+
+#include "thread.h"
+
+static char _dummy_stack[THREAD_STACKSIZE_IDLE];
+
+static void *_dummy_thread(void *arg)
+{
+    (void)arg;
+    while (1) {
+        thread_sleep();
+    }
+
+    return NULL;
+}
+
+void dummy_thread_create(void)
+{
+    thread_create(_dummy_stack, sizeof(_dummy_stack),
+                  THREAD_PRIORITY_IDLE,
+                  THREAD_CREATE_WOUT_YIELD \
+                  | THREAD_CREATE_STACKTEST \
+                  | THREAD_CREATE_SLEEPING,
+                  _dummy_thread, NULL, "dummy");
+}