tests: add test application for SNTP

tests/sntp/Makefile

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+# name of your application
+APPLICATION = sntp
+include ../Makefile.tests_common
+
+BOARD_INSUFFICIENT_MEMORY := chronos msb-430 msb-430h nucleo32-f031 \
+                             nucleo32-f042 nucleo32-l031 nucleo-f030 \
+                             nucleo-f334 nucleo-l053 stm32f0discovery \
+                             telosb weio z1
+
+USEMODULE += sntp
+USEMODULE += gnrc_sock_udp
+USEMODULE += gnrc_ipv6_default
+USEMODULE += auto_init_gnrc_netif
+USEMODULE += gnrc_netdev_default
+USEMODULE += shell
+USEMODULE += shell_commands
+
+# Comment this out to disable code in RIOT that does safety checking
+# which is not needed in a production environment but helps in the
+# development process:
+CFLAGS += -DDEVELHELP
+
+include $(RIOTBASE)/Makefile.include

tests/sntp/README.md

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+About
+=====
+This test application allows you to type `ntpdate <server>` and get the current
+date + the offset in microseconds of the NTP timestamp (seconds from 1900-01-01)
+from the current `xtimer_now_usec64()` back utilizing SNTP. Make sure that
+`<server>` is reachable and has an NTP daemon installed.

tests/sntp/main.c

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+/*
+ * Copyright (C) 2017 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       Tests sntp module.
+ *
+ * @author      Martine Lenders <m.lenders@fu-berlin.de>
+ *
+ * @}
+ */
+
+#include <stdio.h>
+
+#include "shell.h"
+
+static char line_buf[SHELL_DEFAULT_BUFSIZE];
+
+int main(void)
+{
+    shell_run(NULL, line_buf, SHELL_DEFAULT_BUFSIZE);
+    return 0;
+}