dm: core: Add tests for dev_read_alias_highest_id()

It is checking the highest alias ID for eth, gpio, pci, i2c and error
code on non existing alias.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>