tests/driver_vbat: add test for backup battery monitoring

tests/periph_vbat/Makefile

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+include ../Makefile.tests_common
+
+FEATURES_REQUIRED += periph_vbat
+USEMODULE += ztimer ztimer_msec
+
+include $(RIOTBASE)/Makefile.include

tests/periph_vbat/README.md

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+Backup Battery Monitoring Application
+=====================================
+
+This test regularly samples the backup battery voltage and prints the result
+to the serial console.

tests/periph_vbat/app.config.test

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+# this file enables modules defined in Kconfig. Do not use this file for
+# application configuration. This is only needed during migration.
+CONFIG_MODULE_ZTIMER=y
+CONFIG_MODULE_ZTIMER_MSEC=y
+CONFIG_MODULE_PERIPH_VBAT=y

tests/periph_vbat/main.c

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+/*
+ * Copyright (C) 2022 Otto-von-Guericke-Universität Magdeburg
+ *
+ * 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 backup battery monitoring
+ *
+ * @author      Fabian Hüßler <fabian.huessler@ovgu.de>
+ *
+ * @}
+ */
+
+#include <stdio.h>
+#include <inttypes.h>
+
+#include "board.h"
+#include "ztimer.h"
+#include "periph/vbat.h"
+
+int main(void)
+{
+    puts("\nRIOT backup battery monitoring test\n");
+    puts("This test will sample the backup battery once a second\n\n");
+
+    int32_t bat_mv;
+    while (1) {
+        if ((bat_mv = vbat_sample_mv()) < 0) {
+            return 1;
+        }
+        printf("VBAT: %"PRIi32"[mV]\n", bat_mv);
+        ztimer_sleep(ZTIMER_MSEC, 1000);
+    }
+    return 0;
+}