tests: add PCF957X driver test application

tests/driver_pcf857x/Makefile

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+include ../Makefile.tests_common
+
+ifeq (,$(filter pcf857%,$(USEMODULE)))
+    # default expander module
+    USEMODULE += pcf8575
+endif
+
+USEMODULE += shell
+USEMODULE += benchmark
+
+include $(RIOTBASE)/Makefile.include

tests/driver_pcf857x/README.md

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+# Texas Instruments PCF857X I2C I/O expanders test application
+
+## Overview
+
+This test appliation demonstrates the usage of the PCF857X driver interface
+and can be used to test each PCF857X expander I/O pin with shell commands.
+
+The application bases on the test application for GPIO peripheral drivers
+which is under following copyright:
+
+     Copyright (C) 2014,2017 Freie Universität Berlin
+     @author Hauke Petersen <hauke.petersen@fu-berlin.de>
+
+## Usage
+
+To use the test application, compile it with one or more of the pseudomodules
+`pcf8574`, `pcf8574a` or `pcf8575` to enable the driver for your
+expander modules. Please check the default configuration parameters in
+`$(RIOTBASE)/drivers/pcf857x/include/pcf857x_params.h` and adopt them
+if necessary.
+
+     USEMODULE=pcf8575 make -C tests/driver_pcf857x BOARD=...
+
+**Please note:** When no pseudomodule is given, `pcf8575` is used by default.
+
+The usage of the PCF857X low-active open-drain interrupt signal is
+recommended to be able to use external interrupts with the expander I/O
+pins. Add module `periph_gpio_irq` for this purpose and define the
+MCU interrupt pin by parameter `PCF857X_PARAM_INT_PIN`, e.g.
+
+     CFLAGS="-DPCF857X_PARAM_INT_PIN=\(GPIO\(0,6\)\)" \
+     USEMODULE="pcf8575 periph_gpio_irq" make -C tests/driver_pcf857x BOARD=...

tests/driver_pcf857x/include/gpio_ext_conf.h

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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       GPIO extension for Texas Instruments PCF857X I2C I/O expanders
+ * @author      Gunar Schorcht <gunar@schorcht.net>
+ * @file
+ */
+
+#ifndef GPIO_EXT_CONF_H
+#define GPIO_EXT_CONF_H
+
+#if MODULE_EXTEND_GPIO
+
+#include <stddef.h>
+
+#include "extend/gpio.h"
+#include "pcf857x_params.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @brief   Reference to PCF857X device driver struct
+ */
+extern const gpio_ext_driver_t pcf857x_extend_gpio_driver;
+/** @} */
+
+/**
+ * @brief   References to the PCF857X devices
+ */
+extern pcf857x_t pcf857x_dev[];
+
+/**
+ * @brief   GPIO extension list of PCF857X expanders
+ */
+static const gpio_ext_t gpio_ext_list[] =
+{
+    {
+        .driver = &pcf857x_extend_gpio_driver,
+        .dev = (void *)&pcf857x_dev[0],
+    },
+};
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* MODULE_EXTEND_GPIO */
+
+#endif /* GPIO_EXT_CONF_H */
+/** @} */