tests: add PCF957X driver test application

tests/driver_pcf857x/Makefile

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

tests/driver_pcf857x/Makefile.ci

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+BOARD_INSUFFICIENT_MEMORY := \
+    arduino-duemilanove \
+    arduino-leonardo \
+    arduino-nano \
+    arduino-uno \
+    atmega328p \
+    atmega328p-xplained-mini \
+    #

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.
+
+## Compilation
+
+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 modify them
+if necessary. Alternatively, a modified version of this file could be
+placed in the directory of this test application to override it.
+```
+USEMODULE=pcf8575 make -C tests/driver_pcf857x BOARD=...
+```
+**Please note:** When no pseudomodule is given, `pcf8575` is used by default.
+
+To use external interrupts with the expander I/O pins, the PCF857X
+low-active open-drain interrupt signal has to be enabled. Add module
+`pcf857x_irq` for this purpose and define the MCU interrupt pin by
+parameter `PCF857X_PARAM_INT_PIN`, e.g.
+```
+CFLAGS="-DPCF857X_PARAM_INT_PIN=\(GPIO_PIN\(0,6\)\)" \
+USEMODULE="pcf8575 pcf857x_irq" make -C tests/driver_pcf857x BOARD=...
+```
+**Please note:** Since interrupts are handled in the context of a separate
+event thread, enabling interrupts requires more RAM.
+
+## Usage
+
+The test allows to use commands as known from GPIO test application for
+PCF857X expanders:
+```
+> help
+Command              Description
+---------------------------------------
+init_out             init as output (push-pull mode)
+init_in              init as input w/o pull resistor
+init_in_pu           init as input with pull-up
+init_od              init as output (open-drain without pull resistor)
+init_od_pu           init as output (open-drain with pull-up)
+init_int             init as external INT w/o pull resistor
+enable_int           enable or disable gpio interrupt
+read                 read pin status
+set                  set pin to HIGH
+clear                set pin to LOW
+toggle               toggle pin
+bench                run a set of predefined benchmarks
+
+```
+The number of the first PCF857X expander port used by the test application
+is defined by the macro `PCF857X_PORT_0`, which is 16 by default. This value
+can be overridden during compilation, e.g.:
+```
+CFLAGS="-DPCF857X_PORT_0=8" \
+USEMODULE=pcf8575 make -C tests/driver_pcf857x BOARD=...
+```
+Using the port number defined by `PCF857X_PORT_0` and the following port
+numbers, you can apply the command to the PCF857X expander ports. For
+example, the following command initializes I/O pin 7 of the first PCF857X
+expander:
+```
+init_out 16 7
+```
+Commands with port numbers less than `PCF857X_PORT_0` refer to GPIO
+peripheral ports. Thus, both the I/O pins of the PCF857X expanders as well
+as the GPIO peripheral pins of the MCU can be addressed by all commands.

tests/driver_pcf857x/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_PCF857X=y
+
+CONFIG_MODULE_SHELL=y
+CONFIG_MODULE_BENCHMARK=y
+CONFIG_ZTIMER_USEC=y