tests: add test for MODBUS RTU subsystem

The test allow to test both server and client role
simultaneously. Currently only FRDM-K64F board supports
simultaneously server-client test.

To run the test on the board like reel board, where
only one UART interface is available, two device can be used.
One board has to be flashed with project that supports only
the server role and the other with the client role.

Signed-off-by: Johann Fischer <j.fischer@phytec.de>