tests: uart_basic_api: allow USB workqueue/thread to work

Allow UART driver that offload interrupt processing
to workuqueue, like CDC ACM UART to work.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>

tests/drivers/uart/uart_basic_api/src/test_uart_fifo.c

@@ -110,6 +110,8 @@ static int test_fifo_read(void)
 
 	data_received = false;
 	while (data_received == false) {
+		/* Allow other thread/workqueue to work. */
+		k_yield();
 	}
 
 	/* Verify uart_irq_rx_disable() */

tests/drivers/uart/uart_basic_api/src/test_uart_pending.c

@@ -124,6 +124,8 @@ static int test_pending(void)
 	TC_PRINT("Please send characters to serial console\n");
 
 	while (status == WAIT) {
+		/* Allow other thread/workqueue to work. */
+		k_yield();
 		/*
 		 * Wait RX handler change 'status' properly:
 		 * it will change to PASSED or FAILED after

tests/drivers/uart/uart_basic_api/src/test_uart_poll.c

@@ -23,6 +23,8 @@ static int test_poll_in(void)
 	/* Verify uart_poll_in() */
 	while (1) {
 		while (uart_poll_in(uart_dev, &recv_char) < 0) {
+			/* Allow other thread/workqueue to work. */
+			k_yield();
 		}
 
 		TC_PRINT("%c", recv_char);