tests: drivers: uart: async_api: Fix uart_async_read_abort case

Read abort local variable was not reset. If suite was run more than
once on the multiple UARTs it could result in test failure.

Additionally, decreased the baudrate for that test to 9600 to
prolong the transfer and ensure that RX abort happens before transfer
is finised.

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

Author: nordic-krch

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

@@ -625,6 +625,7 @@ static void *read_abort_setup(void)
 
 	uart_async_test_init(idx++);
 
+	test_read_abort_rx_cnt = 0;
 	test_read_abort_rx_buf_req_once = false;
 	failed_in_isr = false;
 	uart_callback_set(uart_dev, test_read_abort_callback, NULL);
@@ -644,13 +645,22 @@ ZTEST_USER(uart_async_read_abort, test_read_abort)
 	 __aligned(sizeof(void *)) uint8_t rx_buf[100];
 	 __aligned(sizeof(void *)) uint8_t tx_buf[100];
 #endif /* NOCACHE_MEM */
+	uint32_t baudrate;
 
 	memset(rx_buf, 0, sizeof(rx_buf));
 	memset(tx_buf, 1, sizeof(tx_buf));
 
 	err = uart_config_get(uart_dev, &cfg);
 	zassert_equal(err, 0);
 
+	/* Lets reduce the baudrate as we want to abort the transfer before it is
+	 * finished and k_timer precision depends on the system timer frequency
+	 * which might be low.
+	 */
+	baudrate = cfg.baudrate;
+	cfg.baudrate = 9600;
+	zassert_ok(uart_configure(uart_dev, &cfg));
+
 	/* Lets aim to abort after transmitting ~20 bytes (200 bauds) */
 	t_us = (20 * 10 * 1000000) / cfg.baudrate;
 
@@ -689,6 +699,8 @@ ZTEST_USER(uart_async_read_abort, test_read_abort)
 	zassert_not_equal(k_sem_take(&rx_buf_coherency, K_NO_WAIT), 0,
 			"All provided buffers are released");
 
+	cfg.baudrate = baudrate;
+	zassert_ok(uart_configure(uart_dev, &cfg));
 }
 
 static ZTEST_BMEM volatile size_t sent;