The shell sample, and a two-board rig for testing on hardware

samples/shell is `rb9704` on the shell with callbacks that print what
arrives, a CRC-32 and the start of each message from the network. Board
overlays for native_sim (the modem a host serial port, a pty or a real
9704 on USB), NUCLEO-U575ZI-Q, XIAO ESP32S3 and ESP32-S3-DevKitC.

tests/rig/bridge copies bytes between a board's USB serial port and a
UART, so two boards wired together stand in for a USB-serial adapter: one
runs the sample, the other bridges its modem UART to the host, where the
fake modem answers. Overlays for a Nucleo and a XIAO, either way round.

tests/rig/bridge/src/main.c

@@ -0,0 +1,57 @@
+/* SPDX-License-Identifier: MIT
+ *
+ * A two-way UART bridge, for the test rig: every byte in on `host` goes out
+ * on `target`, and the other way round. With `host` a board's USB serial
+ * port and `target` a UART wired to another board's modem pins, the fake
+ * modem on the host answers that board as a modem would. The aliases are
+ * the board overlay's. */
+#include <zephyr/kernel.h>
+#include <zephyr/device.h>
+#include <zephyr/drivers/uart.h>
+#include <zephyr/sys/ring_buffer.h>
+
+static const struct device *const host = DEVICE_DT_GET(DT_ALIAS(bridge_host));
+static const struct device *const target = DEVICE_DT_GET(DT_ALIAS(bridge_target));
+
+RING_BUF_DECLARE(to_target, 4096);
+RING_BUF_DECLARE(to_host, 4096);
+
+static void isr(const struct device *dev, void *user)
+{
+	struct ring_buf *out = user;	/* bytes read here go into this ring */
+	const struct device *other = dev == host ? target : host;
+	struct ring_buf *back = out == &to_target ? &to_host : &to_target;
+	uint8_t b[64];
+
+	while (uart_irq_update(dev) && uart_irq_is_pending(dev)) {
+		if (uart_irq_rx_ready(dev)) {
+			int n = uart_fifo_read(dev, b, sizeof(b));
+
+			if (n > 0) {
+				ring_buf_put(out, b, n);
+				uart_irq_tx_enable(other);
+			}
+		}
+		if (uart_irq_tx_ready(dev)) {
+			uint8_t *p;
+			uint32_t n = ring_buf_get_claim(back, &p, 64);
+
+			if (n == 0) {
+				uart_irq_tx_disable(dev);
+			} else {
+				int w = uart_fifo_fill(dev, p, n);
+
+				ring_buf_get_finish(back, w > 0 ? w : 0);
+			}
+		}
+	}
+}
+
+int main(void)
+{
+	uart_irq_callback_user_data_set(host, isr, &to_target);
+	uart_irq_callback_user_data_set(target, isr, &to_host);
+	uart_irq_rx_enable(host);
+	uart_irq_rx_enable(target);
+	return 0;
+}