quad: cleanup circular buffer and add regression tests

quad/sw/modular_quad_pid/src/commands.c

quad/sw/modular_quad_pid/src/communication.c

quad/sw/modular_quad_pid/src/uart_buff.c

quad/sw/modular_quad_pid/src/uart_buff.h

quad/sw/modular_quad_pid/test/.gitignore

quad/sw/modular_quad_pid/test/Makefile

quad/sw/modular_quad_pid/test/test_uart_buff.c

quad/sw/modular_quad_pid/test/xil_types.h