tests: drivers: uart: uart_async_api: update uart_async_api test

add nucleo_c071rb overlay file for test bench purpose.

Signed-off-by: Fabrice DJIATSA <fabrice.djiatsa-ext@st.com>

Author: djiatsaf-st

tests/drivers/uart/uart_async_api/boards/nucleo_c071rb.overlay

@@ -0,0 +1,15 @@
+/* SPDX-License-Identifier: Apache-2.0 */
+
+dut: &usart1 {
+	dmas = <&dmamux1 3 51 STM32_DMA_PERIPH_TX>,
+	       <&dmamux1 4 50 STM32_DMA_PERIPH_RX>;
+	dma-names = "tx", "rx";
+};
+
+&dma1 {
+	status = "okay";
+};
+
+&dmamux1 {
+	status = "okay";
+};