tests: modem: backends: uart: Add UART backend test suite

The UART backend test suite performs 9 iterations of:

1. Open UART backend pipe
2. Transmit 8192 bytes of pseudo random data
3. Receive and validate bytes
4. close UART backend pipe

The test is run on real hardware, with the TX/RX pins
connected to each other to provide loopback functionality.

The test suite has been run on a STM32 and an nRF5340 board.
The test suite tests both the UART interrupt driven and
async APIs.

Signed-off-by: Bjarki Arge Andreasen <bjarki@arge-andreasen.me>