- When orchagent receives a neighbor update with a zero MAC:
    - If the neighbor IP is configured for a specific mux cable port in the MUX_CABLE table in CONFIG_DB, handle the neighbor normally (if active for the port, no action is needed. if standby, a tunnel route is created for the neighbor IP)
    - If the neighbor IP is not configured for a specific port, create a tunnel route for the IP to the peer switch.
        - When these neighbor IPs are eventually resolved, remove the tunnel route and handle the neighbor normally.
- When creating/initializing a mux cable object, set the internal state to standby to match the constructor behavior.

- Various formatting fixes inside test_mux.py
- Remove references to deprecated `@pytest.yield_fixture`
- Add dual ToR neighbor miss test cases:
    - Test cases and expected results are described in `mux_neigh_miss_tests.py`. These descriptions are used by the generic test runner `test_neighbor_miss` function to execute the test actions and verify expected results
    - Various setup fixtures and test info fixtures were added
    - Existing test cases were changed to use these setup fixtures for consistency

Signed-off-by: Lawrence Lee <lawlee@microsoft.com>
Co-authored-by: Sumukha Tumkur Vani <stumkurv@microsoft.com>

Signed-off-by: Lawrence Lee <lawlee@microsoft.com>

Signed-off-by: Lawrence Lee <lawlee@microsoft.com>

Signed-off-by: Lawrence Lee <lawlee@microsoft.com>

…-swss into dualtor-neigh-miss