Implement basic software integration test for OT UART

- The MCU core runs a C program interfacing UART in Caliptra Subsystem to transmit and receive a few dozen bytes of pseudorandom data. 
- The UART shall be its own counterpart, connected in loopback from TX to RX in the integration testbench.
- The UART smoke test from caliptra-rtl shall be reused as much as possible.