tests: build_all: modem: test compilation with connectivity

Ensure WiFi modems still build with the `CONNECTIVITY_WIFI_MGMT`
connectivity backend enabled.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>

tests/drivers/build_all/modem/modem_esp_at.conf

@@ -5,3 +5,5 @@ CONFIG_NETWORKING=y
 CONFIG_NET_SOCKETS=y
 CONFIG_NET_IPV4=y
 CONFIG_WIFI=y
+CONFIG_NET_CONNECTION_MANAGER=y
+CONFIG_NET_CONNECTION_MANAGER_CONNECTIVITY_WIFI_MGMT=y

tests/drivers/build_all/modem/src/main.c

@@ -8,3 +8,15 @@ int main(void)
 {
 	return 0;
 }
+
+#ifdef CONFIG_CONNECTIVITY_WIFI_MGMT_APPLICATION
+
+#include <zephyr/net/conn_mgr_connectivity_impl.h>
+#include <zephyr/net/conn_mgr/connectivity_wifi_mgmt.h>
+
+/* Bind L2 connectity APIs. */
+static struct conn_mgr_conn_api conn_api = { 0 };
+
+CONN_MGR_CONN_DEFINE(CONNECTIVITY_WIFI_MGMT, &conn_api);
+
+#endif /* CONFIG_CONNECTIVITY_WIFI_MGMT_APPLICATION */