Enable dynamic BACnet POSIX files for example (#374)

* Enable dynamic BACnet POSIX files for example

* Add basic unit test for BACnet File object.

* Use example file pathname in demo app.

Co-authored-by: Steve Karg <skarg@users.sourceforge.net>

test/bacnet/basic/object/mock/tsm_mock.c

@@ -0,0 +1,26 @@
+/**
+ * @file
+ * @brief mock TSM handler functions
+ * @author Steve Karg
+ * @date January 2023
+ *
+ * SPDX-License-Identifier: MIT
+ */
+#include <ztest.h>
+#include <bacnet/basic/tsm/tsm.h>
+
+bool tsm_get_transaction_pdu(
+    uint8_t invokeID,
+    BACNET_ADDRESS * dest,
+    BACNET_NPDU_DATA * ndpu_data,
+    uint8_t * apdu,
+    uint16_t * apdu_len)
+{
+    (void)invokeID;
+    (void)dest;
+    (void)ndpu_data;
+    (void)apdu;
+    (void)apdu_len;
+
+    return false;
+}