Refactor Drop EtherCat packet test

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

This includes:
1. Add a method to send packet and expect counter increases.
2. Inline the packet construction code in the test method.

Test: atest NetworkStaticLibHostPythonTests
Test: m connectivity_multi_devices_snippet && \
      atest CtsConnectivityMultiDevicesTestCases
Bug: 350880989
Change-Id: Iadcfd97740033306af1b6a474004cbf0699ddd1b
4 files changed