tests: add SHT3X sensor driver test application

tests/driver_sht3x/Makefile

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+include ../Makefile.tests_common
+
+USEMODULE += sht3x
+
+include $(RIOTBASE)/Makefile.include

tests/driver_sht3x/README.md

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+# Sensirion SHT30/SHT31/SHT35 Humidity and Temperature Sensors
+
+## About
+This is a manual test application for the SHT3x driver.
+
+## Usage
+This test application will initialize the sensor with the following parameters:
+
+- Periodic Measurement with 2 measurements per second
+- High Repeatability
+
+After initialization, the sensor reads the temperature and humidity every second
+and prints them to the STDOUT.

tests/driver_sht3x/main.c

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+/*
+ * Copyright (C) 2018 Gunar Schorcht
+ *
+ * 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
+ * @brief       Test application for the Sensirion SHT30/SHT31/SHT35 device driver
+ * @author      Gunar Schorcht <gunar@schorcht.net>
+ * @file
+ * @{
+ * @}
+ */
+
+#include <stdio.h>
+#include <string.h>
+
+#include "xtimer.h"
+#include "sht3x.h"
+#include "sht3x_params.h"
+
+int main(void)
+{
+    sht3x_dev_t dev;
+    int res;
+
+    puts("SHT3X test application\n");
+
+    printf("+------------Initializing------------+\n");
+
+    if ((res = sht3x_init(&dev, &sht3x_params[0])) != SHT3X_OK) {
+        puts("Initialization failed\n");
+        return 1;
+    }
+    else {
+        puts("Initialization successful\n");
+    }
+
+    printf("\n+--------Starting Measurements--------+\n");
+
+    while (1) {
+        int16_t temp;
+        int16_t hum;
+
+        if ((res = sht3x_read(&dev, &temp, &hum)) == SHT3X_OK) {
+            printf("Temperature [°C]: %d.%d\n"
+                   "Relative Humidity [%%]: %d.%d\n"
+                   "+-------------------------------------+\n",
+                   temp / 100, temp % 100,
+                   hum / 100, hum % 100);
+        }
+        else {
+            printf("Could not read data from sensor, error %d\n", res);
+        }
+        xtimer_usleep(1000000);
+    }
+
+    return 0;
+}