tests/mutex_unlock_and_sleep: reduce number of iterations on boards

Reduce number of iterations, it allows taking less than 3 seconds on wsn430.

Tested on wsn430 and iotlab-m3 to check that speed was not too fast to read when
executing term.

tests/mutex_unlock_and_sleep/main.c

@@ -27,7 +27,11 @@ static volatile int indicator;
 static kernel_pid_t main_pid;
 static char stack[THREAD_STACKSIZE_DEFAULT];
 
+#ifdef BOARD_NATIVE
 static const unsigned KITERATIONS = 100;
+#else
+static const unsigned KITERATIONS = 10;
+#endif
 
 static void *second_thread(void *arg)
 {