Temporarily fix and workaround the Lua HW tests on OSX buildhost

It seems like OSX doesn't like 460800 baudrate and XON/XOFF flow control. It
ends with RS232_ERR_CONFIG. Need to investigate it further later.

I've also hardcoded serial device name for OSX for now as I've one setup
running between two Linux build slaves and one OSX and I can't figure out how
to handle this properly without splitting the build config (I would like to
avoid doing that).

Signed-off-by: Petr Štetiar <ynezz@true.cz>