TEST: Fix net_test_stock_rfcomm unit test

* Properly mock updated L2CAP registeration method by including the
  filtered btsnoop option
* Properly mock btsnoop module by creating a separate file to link with
  the test executable

Fixes: 123361801
Test: atest net_test_stack_rfcomm
Change-Id: I89ca875cbcc941feddd7c501fa1b7e14f8d14794