tests: drivers: uart: uart_async_api: Tweak test_read_abort test

Test was staring a TX transfer and aborting it after 300 us from
the timer handler. Test was assuming that ongoing transfer will
not finish in those 300 us. This might not be true for higher
baudrates. Instead of using fixed timeout, a value is calculated
from the baudrate and targets to abort the transfer after approx.
20 bytes of 95 byte long transfer.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>