Add structure of eth_tester files, without RTL.

libraries/io/eth/hdllib.cfg

libraries/io/eth/src/vhdl/eth_tester.vhd

libraries/io/eth/src/vhdl/eth_tester_rx.vhd

libraries/io/eth/src/vhdl/eth_tester_tx.vhd

libraries/io/eth/tb/vhdl/tb_eth_tester.vhd

libraries/io/eth/tb/vhdl/tb_tb_eth_tester.vhd