Fix net_test_hci unit tests

* Added new function API to hci_layer: hci_layer_cleanup_interface()
  It can be used to cleanup after hci_layer_get_interface()
  or hci_layer_get_test_interface()
* Fixed the unit tests inside HciLayerTest by removing usage of
  allocation_tracker_reset() and adding hci_layer_cleanup_interface() .
  The former was used as a hack to bypass the internal memory allocation
  of hci_layer_get_test_interface(). The latter is used to properly
  cleanup such allocation.

Bug: 26014740
Change-Id: I498b996a931c59dd3b4258f1a5741cfa6d302e42