UART loopback sample/test, designed for high data rate testing

[nordicjm]

Is your feature request related to a problem? Please describe.
At present, there is an "echo bot" sample in zephyr for the UART peripheral, this is fine as a simple sample but cannot really be used as much more than that, it changes messages that are received, it is not optimised for high throughput (and actually falls over very fast) and this is far worse if a USB CDC backend is used.

I have my own UART test utility and when I made some minor modifications to echo bot, was seeing a 48% error rate of received data.

Describe the solution you'd like
A sample that loops back the RX UART data to TX, with a configuration that is designed for high throughput, without appending/prepending anything to the data, so that it can also be used as a driver test sample (or test) application to check what the sustained data rate is, and error rate.

Describe alternatives you've considered
None

[dcpleung]

I think the echo_bot is reading from FIFO 1 byte at a time... so it might be the bottleneck.