- If unable to resolve a neighbor on a dual ToR system, write a neighbor table entry for that neighbor with a zero MAC

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

- When receiving 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.

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

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

- Insert newline before open braces
- Use MacAddress boolean conversion

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

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

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

- Move standalone tunnel neighbor handling to completely separate code path

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

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

- Add dual ToR neighbor miss test cases:
    - Create generic test runner function that executes test
      steps described in mux_neigh_miss_tests.py
    - Add neighbor miss test case with missing PEER_SWITCH config
- Create setup fixtures for dual ToR DB entries
    - CONFIG_DB: VLAN, MUX_CABLE, PEER_SWITCH
    - APPL_DB: TUNNEL_DECAP_TABLE
- Refactor existing tests to use setup fixtures for consistency
- Create ASIC_DB and APPL_DB verification functions
    - Verify APPL_DB NEIGH_TABLE entry presence/absence
    - Verify ASIC_DB ROUTE_ENTRY and NEIGH_ENTRY entry presence/absence
- Create various fixtures to generate test information

Signed-off-by: Lawrence Lee <lawlee@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>