[tests] uart_parity_break and uart_tx_rx_test require two UARTs

### Description

Those tests were originally written for Earlgrey, which has 4 UARTs. Darjeeling only has a single UART, thus making them incompatible. Those tests use the second UART solely for logging.

@pamaury proposed to use the SPI console as an alternative logging facility.

via https://github.com/lowRISC/opentitan/pull/28316

Start with the uart_parity_break and uart_tx_rx_test tests and identify where their second UART is used for logging. Review the SPI console proposal, then adapt the tests so they can run on Darjeeling with its single UART while retaining their existing test behavior.

Written by the indexing model from the issue text.