Modify backpressure test to something sensible

[shuchitak]

https://xmosjira.atlassian.net/browse/AP-420

The backpressure test made no sense to me, so I've tried to modify it to atleast test something. I ran the test and logged the callback functions' delay values at which the backpressure breaks the LRCLK timing and used these values as the test pass threshold. This way, if there's ever a change in the library that reduces the allowed backpressure from the callback functions, we'd be able to catch that (once the CI runs are fixed!).
Also, for 768KHz, even a delay of 0 was breaking the LR timing so I removed 768KHz from the list of sample rates we tested.

The test in its current form passes consistently on my Mac.

pytest lib_i2s/test_backpressure.py -s

[ACascarino]

Hitting request changes so I can be notified when the work is done - as discussed, would be good if this test could actually guarantee some level of operation i.e. writing to and reading from memory within the callback