test: i2c: Add i2c_slave_api test

Add a new test to make I2C transfers between an I2C controller and
an I2C slave driver, either by:
- Using a virtual I2C driver
- Using a physical link and 2 physical controllers

The Physical test uses :
- Two EEPROM Slave drivers each connected to one I2C controller
- Both I2C controllers also act as master and take turns writing
to the opposite EEPROM Slave.
All of this connected physically on the board.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Daniel Wagenknecht <wagenknecht@clage.de>