tests: modem: backends: uart: add hw flow control option

Extend test modem uart backend test suite to support testing hw
flow control, which is performed by using a small receive
buffer for the modem uart backend, and slowing down the read
of received data, ensuring the buffer will be overrun if hw flow
control is not working.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>