tests: drivers: uart: uart_async_api: fix error on test_write_abort

With the changes made in PR #76061, only on the F429ZI platform, in
the test_write_abort test,the number of received data (2) is not equal
to the number of sent data (1). It looks like uart_tx_abort is done very
quickly, and the callback doesn't have enough time to deal with it.
Adding a small delay solves the problem and doesn't affect results on
other platforms.

Signed-off-by: Fabrice DJIATSA <[email protected]>

tests/drivers/uart/uart_async_api/src/test_uart_async.c

@@ -712,6 +712,7 @@ ZTEST_USER(uart_async_write_abort, test_write_abort)
 	if (sent != 0) {
 		zassert_equal(k_sem_take(&rx_rdy, K_MSEC(100)), 0,
 			      "RX_RDY timeout");
+		k_sleep(K_MSEC(30));
 		zassert_equal(sent, received, "Sent is not equal to received.");
 	}
 	uart_rx_disable(uart_dev);