Converted device object test to use common read-write property test. Extended the basic BACnet device object example API. (#1106)

* Converted device object test to use common read-write property test. Extended the basic BACnet device object example API.

* Created BACnet/IP and COV test mocks to enable device object testing with less dependencies.

test/bacnet/basic/object/test/cov_mock.c

@@ -0,0 +1,17 @@
+/**
+ * @file
+ * @brief mock for COV functions
+ * @author Steve Karg
+ * @date September 2025
+ * @copyright SPDX-License-Identifier: MIT
+ */
+#include <zephyr/ztest.h>
+#include <bacnet/bacdef.h>
+#include <bacnet/basic/services.h>
+
+int handler_cov_encode_subscriptions(uint8_t *apdu, int max_apdu)
+{
+    (void)apdu;
+    (void)max_apdu;
+    return 0;
+}