drivers: move eeprom_slave driver to tests directory

This device isn't an actual hardware driver: it's a virtual EEPROM
that stores data in an instance-specific RAM buffer, with the data
exposed on an I2C bus as a I2C follower (slave) device that can be
controlled by another device acting as a leader (master) on that same
bus.

As such it's a reasonable example of how to write an I2C follower
driver, but it's not clear that it has a real use in applications.  A
Zephyr application that needs to emulate an EEPROM in a real-world
system would be unlikely to provide its data from a RAM buffer.

The sole in-tree reference is in the i2c_slave_api test, so move the
driver implementation into that test.

The Kconfig and hierarchy are being left in place until it is more
clear how this functionality should be selectable within Zephyr.  The
I2C_SLAVE symbol has been converted from menuconfig to config to
eliminate a Kconfig style diagnostic.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>