uartReadTimeout - Test if UART is not transmitting

Assert that nothing is received on a UART port. If sendText is given, it is sent first; the step then passes if no data arrives before the timeout elapses, and fails if any data is received.

Usage:

- command: uartReadTimeout <uartPort> [<seconds> [<sendText>]]
  [retry: <retryCount>]

Argument	Description
uartPort	UART port name: `UART0`, `UART1`, `UART2`, etc.
seconds	Number of seconds to wait for incoming data. If omitted, the timeout in effect from a prior step’s `timeout:` is used.
sendText	Text to send before waiting. Prefix with `hex` to interpret as hex bytes.
retryCount	Number of times to retry the test step (optional)

The RX buffer is always flushed immediately before the silence-check begins, discarding any data received prior to the step (and prior to sending sendText, if given).