Move apf test to a standalone file

This is a no-op refactoring, which is preparation for follow-up
changes to add more apfv4 tests.

This includes:
  1. Add a entry point for multi-device tests to collect
     test cases from different files.
  2. Move multi-device initialization code to a base class.
  3. Make different test classes inherit the base class.

Test: m connectivity_multi_devices_snippet && \
      atest CtsConnectivityMultiDevicesTestCases
Bug: 350880989
Change-Id: I294c85d8e09633bbf02ba1543b9f2ea3e988e20d
5 files changed