tests: drivers: uart: uart_async_api: Move rx abort to timeout

Intention of the test is to abort rx few bytes after start of
transmission (before it is completed). Previously, thread was busy
waiting after start of TX and aborted RX from that context. However,
it may happen that CPU is busy handling UART transfer and
k_busy_wait prolongs beyond full transfer which results in test
failure.

Move rx_abort to k_timer timeout which is run in interrupt
context.

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