tests: drivers: uart: async_dual: Optimize test data handling

Stress test is executed on CPUs with slow clock (16MHz). Handling of test
data in UART_RX_RDY event is optimized to reduce time spent in the
interrupt context. Since payload is always a decrementing sequence, fixed
array is used to compare memory which allows to use standard memcmp
instead of byte by byte comparison.

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