[nrf fromlist] tests: drivers: uart: async_api: Improve variable buffer length test

Test was skipped if baudrate was too high. Test can actually
reconfigure the DUT device to use lower baudrate and run the test.
Initial baudrate is restored once test is completed.

Upstream PR #: 106261

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

Author: nordic-krch

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

@@ -1149,17 +1149,23 @@ static ZTEST_BMEM uint8_t tx_buffer[VAR_LENGTH_TX_BUF_SIZE];
 ZTEST_USER(uart_async_var_buf_length, test_var_buf_length)
 {
 	struct uart_config uart_cfg;
+	uint32_t baudrate;
 
 	zassert_equal(uart_config_get(uart_dev, &uart_cfg), 0);
+	baudrate = uart_cfg.baudrate;
 	if (uart_cfg.baudrate > CONFIG_VAR_LENGTH_BUFFER_TEST_BUADRATE_LIMIT) {
-		ztest_test_skip();
+		uart_cfg.baudrate = CONFIG_VAR_LENGTH_BUFFER_TEST_BUADRATE_LIMIT;
+		zassert_ok(uart_configure(uart_dev, &uart_cfg));
 	}
 
 	for (size_t buf_len = 1; buf_len < CONFIG_VAR_LENGTH_BUFFER_MAX_SIZE; ++buf_len) {
 		for (size_t tx_len = 1; tx_len < VAR_LENGTH_TX_BUF_SIZE; ++tx_len) {
 			test_uart_async_var_buf(buf_len, tx_len);
 		}
 	}
+
+	uart_cfg.baudrate = baudrate;
+	zassert_ok(uart_configure(uart_dev, &uart_cfg));
 }
 
 ZTEST_SUITE(uart_async_single_read, NULL, single_read_setup,