Avoid DHCP stress count file collisions

[ashutosh-agrawal]

Summary

• pass a unique DHCP stress count-file path into each PTF run
• derive per-interface tcpdump logs from that unique path
• keep the legacy count-file fallback for direct PTF invocations

Why

Concurrent parametrized stress runs can otherwise share /tmp paths by packet type and interface, allowing one run to overwrite or remove another run's count/log files.

Validation

• python3 -m py_compile ansible/roles/test/files/ptftests/py3/dhcp_relay_stress_test.py tests/dhcp_relay/test_dhcp_relay_stress.py
• git diff --check -- ansible/roles/test/files/ptftests/py3/dhcp_relay_stress_test.py tests/dhcp_relay/test_dhcp_relay_stress.py