Add initial test case for Modbus Server Container

Includes new test group for the modbus server, alongside the initial
test case where the modbus server is coneceted to by a client on the
github action's runner. Currently, the github actions runner connects to
the modbus server to read and write to the LED and Button register(s).
Will need to update test case as registers are implemented in the modbus
server container.

This commit also includes a refactoring of parts of build.yml in the
github actions to allow the actions runner to flash the board with the
modbus server. Also changes the overall workflow slighly in order to
make future container testing on the b_u585i_iot02a board easier to implement.

Signed-off-by: Matthew Gee <mgee@iol.unh.edu>

Author: mgeeIOL