device_iot_config: add unit tests

This patch provides unit tests for the public and
internal methods of the device_iot_config feature.

The callers of this API are not tested.

Bug: 261319328
Tag: #feature
Test: atest bluetooth_test_gd
BYPASS_LONG_LINES_REASON: Bluetooth likes 120 lines

Change-Id: I3e7ee29bba0746d66e77872eb96b6c3e60be0b74